The specific gravity is a measure of whether a mineral is heavy or light. It is a comparison of the weight of a piece of the mineral with the weight of an equal volume of water. The mineral quartz, for example, has a specific gravity of 2.65. This means that a piece of quartz is a little more than 2½ times as heavy as an equal volume of water. Accurate measurements of specific gravity can be made in a laboratory. You can, however, learn to estimate specific gravities just by lifting various minerals and judging whether they are heavy or light.
This is a property that depends on the chemical composition of the mineral. Carbonate minerals, which contain (in addition to at least one other element) three parts of oxygen and one part of carbon, can be tested with dilute hydrochloric acid. When a drop or two of this acid is put on a carbonate mineral such as calcite (calcium carbonate, CaCO₃), the acid begins to bubble and fizz. The fizzing or effervescence is caused by the carbon dioxide gas that is formed when the acid and mineral come in contact with each other. This test is also helpful in identifying rocks, such as limestone and marble, that contain carbonate minerals.
Beautiful mineral deposits occur in some natural caves. Deposits that look like icicles, called stalactites, are found hanging from the ceiling of a cave. Other deposits, stalagmites, are like the stalactites except that they jut upward from the floor. Columns are formed from stalactites and stalagmites that have joined together. In addition, some caves contain sheet-like deposits that are spread along the ceiling, floor, and walls. These deposits are called flowstone. Calcite is one of the minerals that commonly form cave deposits.
Just a few of the caves in Texas contain these deposits. They occur mostly in the limestone rocks that are south and southwest of the Llano uplift area of central Texas. Some of the commercial caves that contain good examples of calcite deposits are located near Boerne in Kendall County and near Sonora in Sutton County. Calcite deposits also occur in Longhorn Cavern, a large cave located in the Longhorn Cavern State Park of Burnet County. These caves were formed by underground waters that moved through cracks and pores in the limestone rocks and dissolved passageways in them. After the cave passages were made, water containing dissolved calcium carbonate dripped into the cave. As it evaporated, this water left behind a deposit of calcium carbonate—the mineral calcite.
You can better understand how the cave deposits are formed by watching icicles grow in wet, freezing weather. First, small hanging drops of water freeze, and a small icicle forms. Then, as more water drips over it and freezes, the icicle grows longer and wider. Some of the water drips completely over the icicle and falls to the ground. There, it either freezes into a sheet of ice, or it begins to build upward to form an upside-down icicle. The water dripping down in the caves evaporates instead of freezing, and in doing so it leaves behind a deposit of calcite.
Calcite stalactites and stalagmites in the Caverns of Sonora, Sutton County, Texas. Photograph courtesy of the Travel and Information Division of the Texas Highway Department.
Limestone, shale, and other sedimentary rocks commonly have scattered throughout them masses of other rocks and minerals, such as limonite, chert, and pyrite. These masses are called concretions. Concretions may be round or oval, or they may have odd, irregular shapes. They—such as some of the limonite concretions of east Texas—even may look like gourds or sweet potatoes. Concretions generally are harder than the surrounding rocks. Some are smaller than peas, but others are several feet wide. (The word nodule is used to describe small, rounded concretions as well as other small, rounded mineral occurrences.)
It is believed that some concretions form at the same time as the rocks in which they occur. Other concretions develop after the rocks themselves have formed. These are deposited by underground water that contains dissolved mineral matter. The water seeps through the rocks and deposits mineral matter around an object in the rock, such as a fossil or a grain of sand, to form a concretion.
Geodes are rounded, generally hollow masses that occur mostly in limestones. They are scattered through the rocks and can be lifted or dug out. Some geodes are as small as walnuts, and some are as large as basketballs. Most of them have a rough, dull-looking outer surface. If you break geodes open, you will find that many are lined with beautiful crystals of calcite, celestite, or quartz that point inward toward the hollow center.
Calcite geode found in Lower Cretaceous strata of western Travis County, Texas.
It is thought that a geode forms when water, carrying dissolved mineral material, seeps into a cavity in the rock, then deposits the mineral material as a lining in the cavity. This lining becomes the outer part of the geode. Thus a geode—unlike a concretion, which grows from the center outward—forms from outside to inside.
Some of the Lower Cretaceous limestone rocks of Travis, Williamson, and Lampasas counties contain calcite and celestite geodes. Celestite geodes have also been found in Permian rocks in parts of Coke, Fisher, and Nolan counties.
Petrified wood from Texas Gulf Coastal Plain.
We often find some minerals occurring as petrified wood. (Petrified wood includes silicified wood, opalized wood, agatized wood, and carbonized wood.) Petrified wood forms when plant material, such as a tree or a bush, is replaced by a mineral. It is formed by underground water carrying dissolved mineral matter. As this water seeps through sediments in which the plants are buried, it gradually deposits agate, chalcedony, calcite, opal, chalcocite, or some other mineral in the place of each fiber of the wood. By this slow change from plant to mineral matter, the original shape and structure of the wood remain unchanged.
Petrified wood is commonly found in some of the Tertiary, Permian, and Lower Cretaceous rocks of Texas. (See Opal, Quartz, Copper Minerals, pp. 78, 84, 52).
Perhaps you would like to start your own collection of rocks and minerals. For this purpose you will need a hammer (a prospector’s hammer with a pick on one end of it is a good tool), some newspapers to wrap around the specimens to keep them from breaking, and a cloth bag in which to carry the specimens.
Prospector’s hammer.
Before you start to collect, be sure to ask the owner’s permission to go on his property. If he agrees to let you come on his land, be careful about closing gates, and do not leave holes into which his livestock might step and be injured. Look out for snakes. Plenty of rattlers, copperheads, and moccasins are still left in Texas. And, incidentally, collecting is not allowed in State or National parks.
To identify the rocks and minerals that you collect, you probably will need several articles with which to make simple tests. The following can be easily obtained:
1. A pocket knife, a copper penny, a piece of window glass, a steel file, and a piece of quartz to test the hardness. If you prefer to use a group of minerals of known hardness, such as those of Mohs scale described on pages 16-17, you can either collect your own or buy a prepared set from a mineral supply house.
2. A streak plate to test the color of the mineral’s streak. Mineral streak plates can be purchased, or a piece of unglazed tile can be used.
3. A magnifying glass to examine small cleavage surfaces, crystals, and rock grains. A number of different kinds can be bought, from the simple reading glass to the precisely made hand lens. A lens with ten-power magnification is good for general use.
4. A small magnet to test whether or not a mineral is magnetic.
5. Dilute (10%) hydrochloric acid (HCl), also known as muriatic acid, to test carbonate rocks and minerals. You can buy a small bottle at a drug store. Be extremely careful in handling this acid, and keep it away from small children—it is a POISON. If you spill any on yourself, it will burn your skin and eat holes in your clothes.
Hand lens.
The rock and mineral identification charts on pages 24-41 will help you to make the simple identification tests in a methodical way.
It is a good idea to have some system of labeling your rock and mineral specimens. Some collectors carry note paper with them on field trips. Then they can write down the location and, if possible, the name of the rock or mineral. This information is either wrapped with the specimen or stuck to it with tape. One way to label large collections is to put a small spot of paint or fingernail polish on each of the rock and mineral specimens. When the paint has dried, a number can be written on it in black India ink. Then, on a file card, the name and the number of the specimen can be written, together with the place where it was found, the date of collection, and the name of the collector.
To help you identify them, various Texas rocks and minerals are listed together in the following charts according to properties that they have in common. Although useful, the identification charts may not always give you perfect results. For example, hardness, which is used as a guide, is not to be completely relied upon in the identification of rocks.
The charts on the following pages pertain only to the rocks and minerals that are described in this book. It is quite possible that you will find rocks and minerals in Texas that are not included in these charts.
If you find a rock or a mineral that you are unable to identify, you can check your local library for reference books that may aid you (several such references are noted on pages 100-101). If you need further help, possibly the science teacher at a nearby public school will be able to identify the specimen for you. Or if a college or university is located in your area (especially one that has a department of geology), you can obtain help there. In Texas, the Bureau of Economic Geology is a mineral information center. Most other states have similar geological research and public-service organizations. Other sources of information might be the gem and mineral societies that are found in a number of communities. Many of the members of these organizations are experts in the identification of rocks and minerals.
In the mineral identification charts (pp. 26-38), the minerals have been grouped, first of all, on the basis of luster: the first group includes the minerals that appear metallic and almost metallic (submetallic); the second group includes those that appear nonmetallic. Next, the minerals have been arranged within the two groups according to color.
After you have determined the luster and the color of an unknown mineral, turn to the Key to Mineral Identification Charts on page 25. It will direct you to the proper mineral chart.
Mineral Charts 1 through 5, which include the minerals of various colors with metallic and submetallic lusters, are subdivided according to the hardness of the minerals. To determine the hardness of a mineral that has one of these lusters, you can make the following tests:
1. Will the mineral readily leave a mark on paper?
2. If it will not readily leave a mark on paper, will an ordinary pocket knife scratch it?
3. Is it too hard to be scratched by an ordinary pocket knife?
Mineral Charts 6 through 15 are for the nonmetallic minerals of various colors. They, too, are subdivided according to the hardness of the minerals, as follows:
1. Can the mineral be scratched by a fingernail?
2. If it cannot be scratched by a fingernail, can it be scratched by a copper penny?
3. If it cannot be scratched by a copper penny, can it be scratched by an ordinary pocket knife?
4. If it cannot be scratched by an ordinary pocket knife, can it be scratched by a piece of quartz?
5. Is it too hard to be scratched by quartz?
When the luster, color, and hardness of a mineral have been determined, you may find that several minerals on the charts fit the description. To narrow your choice, you can then test other properties of the mineral. Notice the “remarks” column on the charts. In it, is mentioned anything that is distinctive about the mineral.
For more complete mineral identification lists and tables, you can use textbooks, such as Dana’s Manual of Mineralogy, revised by C. S. Hurlbut, Jr., or Mineralogy, by E. H. Kraus, W. F. Hunt, and L. S. Ramsdell.
If the mineral has a metallic or submetallic luster,
| and is: | Consult Mineral Chart |
| white | 1 |
| gray | 2 |
| yellow | 3 |
| brown | 4 |
| black | 5 |
If the mineral has a nonmetallic luster,
| and is: | Consult Mineral Chart |
| white | 6 |
| gray | 7 |
| yellow | 8 |
| brown | 9 |
| black | 10 |
| green | 11 |
| blue | 12 |
| red or pink | 13 |
| purple or violet | 14 |
| colorless | 15 |
| Chart No. | Mineral | Streak | Remarks | Hardness | |
| 1. | METALLIC luster, WHITE color | ||||
|---|---|---|---|---|---|
| A. | Does not readily leave mark on paper but can be scratched by ordinary pocket knife | ||||
| Native silver | Shiny silver white, unless tarnished | Silver-white color that tarnishes to gray, black, or yellowish brown; heavy; can be flattened when hit with hammer | 2½-3 | ||
| 2. | METALLIC or SUBMETALLIC luster, GRAY color | ||||
| A. | Will leave mark on paper | ||||
| Argentite | Shiny, blackish to lead gray | Lead-gray color that tarnishes to dull black; knife cuts it smoothly; heavy; may occur as masses and coatings | 2-2½ | ||
| Galena | Grayish black | Shiny lead-gray color; heavy; cube-shaped fragments and crystals | 2½ | ||
| Graphite | Black | Steel-gray color; greasy feel; very soft; splits into thin flakes | 1-2 | ||
| B. | Does not readily leave mark on paper but can be scratched by ordinary pocket knife | ||||
| Chalcocite | Grayish black | Shiny lead-gray color that tarnishes to dull black; knife cuts it smoothly; may have black sooty coating; commonly occurs as compact or granular masses | 2½-3 | ||
| Hollandite | Black | Silvery-gray color; may occur as rounded masses | 4-6 | ||
| C. | Cannot be scratched by ordinary pocket knife | ||||
| Braunite | Steel gray or black | Dark steel-gray color and submetallic luster | 6-6½ | ||
| Hematite | Dark reddish brown | Steel-gray color; commonly occurs as granular or compact masses; shiny, scaly variety is specular hematite; notice streak | 5½-6½ (may be softer) | ||
| Hollandite | Black | Silvery-gray color; may occur as rounded masses | 4-6 | ||
| 3. | METALLIC luster, YELLOW color | ||||
| A. | Does not readily leave mark on paper but can be scratched by ordinary pocket knife | ||||
| Chalcopyrite | Greenish black | Brass-yellow or golden-yellow color that may tarnish and show rainbow-like colors; commonly massive; notice streak | 3½-4 | ||
| Gold | Shiny golden yellow | Shiny yellow color; extremely heavy; flattens when hit with hammer; notice streak | 2½-3 | ||
| B. | Cannot be scratched by ordinary pocket knife | ||||
| Pyrite | Black, greenish black, or brownish black | Shiny, pale golden-yellow or brass-yellow color that may tarnish; occurs as grains, as masses, or as cubes or other crystal shapes; notice hardness and streak | 6-6½ | ||
| 4. | METALLIC or SUBMETALLIC luster, BROWN color | ||||
| A. | Does not readily leave mark on paper but can be scratched by ordinary pocket knife | ||||
| Limonite | Rusty yellowish brown | Dark-brown color; some specimens have a shiny black surface; notice streak | 5-5½ | ||
| B. | Cannot be scratched by an ordinary pocket knife | ||||
| Cassiterite | Pale brown, pale yellow or white | Brown; submetallic; heavy; notice streak | 6-7 | ||
| Hematite | Dark reddish brown | Dark brown color; commonly occurs as granular or compact masses; notice streak | 5½-6½ (may be softer) | ||
| Limonite | Rusty, yellowish brown | Dark brown color; some specimens have a shiny black surface; notice streak | 5-5½ | ||
| 5. | METALLIC or SUBMETALLIC luster, BLACK color | ||||
| A. | Will leave mark on paper | ||||
| Argentite | Shiny, blackish to lead grey | Lead-gray color that tarnishes to dull black; knife cuts it smoothly; heavy; may occur as masses and coatings | 2-2½ | ||
| Graphite | Black | Greasy feel; very soft; splits into thin flakes | 1-2 | ||
| Pyrolusite | Black | Very soft; will soil fingers; may be powdery | 1-2 | ||
| B. | Does not readily leave mark on paper but can be scratched by an ordinary pocket knife | ||||
| Chalcocite | Grayish black | Shiny lead-gray color that tarnishes to dull black; knife cuts it smoothly; may have a black sooty coating; commonly occurs as compact or granular masses | 2½-3 | ||
| Hollandite | Black | May occur as rounded masses | 4-6 | ||
| Limonite | Rusty, yellowish brown | Some specimens have shiny black surface; notice streak | 5-5½ | ||
| C. | Cannot be scratched by an ordinary pocket knife | ||||
| Braunite | Steel gray or black | Luster is submetallic | 6-6½ | ||
| Cassiterite | Pale brown, pale yellow, or white | Submetallic luster; heavy; notice streak | 6-7 | ||
| Hematite | Dark reddish brown | Notice streak; commonly occurs as granular or compact masses | 5½-6½ (may be softer) | ||
| Hollandite | Black | May occur as rounded masses | 4-6 | ||
| Limonite | Rusty yellowish brown | Some specimens have shiny black surface; notice streak | 5-5½ | ||
| Magnetite | Black | Fragments cling to a magnet | 6 | ||
| Pitchblende | Brownish black | Brownish black, greenish black, or black; radioactive; heavy; may appear dull or greasy | 5½ | ||
| 6. | NONMETALLIC luster, WHITE color | ||||
| A. | Can be scratched by a fingernail | ||||
| Cerargyrite | Shiny white or gray | Appears waxy; knife cuts it smoothly; turns violet brown to black when exposed to light | 1-1½ | ||
| Gypsum | White | Soft; occurs as crystals or as fibrous, granular, compact, or earthy masses | 2 | ||
| Talc | White | Knife cuts it smoothly; feels soapy or greasy; splits into thin flakes | 1 | ||
| B. | Cannot be scratched by a fingernail but can be scratched by a copper penny | ||||
| Anhydrite | White | Commonly occurs as sugary-looking masses | 3-3½ | ||
| Barite | White | Rather heavy; cleavage fragments may be flat and slab-like | 3-3½ | ||
| Calcite | White | Dilute hydrochloric acid fizzes on calcite; perfect cleavage in 3 directions gives rhomb-shaped fragments | 3 | ||
| Celestite | White | Not quite as heavy as barite; crystals commonly prism-shaped or flat-looking; some cleavage fragments are flat and slab-like | 3-3½ | ||
| Halite | White | Salty taste; dissolves in water; cube-shaped cleavage fragments | 2½ | ||
| C. | Cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife | ||||
| Anhydrite | White | Commonly occurs as sugary-looking masses | 3-3½ | ||
| Barite | White | Rather heavy; cleavage fragments may be flat and slab-like | 3-3½ | ||
| Celestite | White | Not quite as heavy as barite; crystals commonly prism-shaped or flat-looking; some cleavage fragments are flat and slab-like | 3-3½ | ||
| Dolomite | White | Commonly occurs as granular masses and as rhomb-shaped crystals; dilute hydrochloric acid may fizz slightly on dolomite | 3½-4 | ||
| Fluorite | White | Cleavage in 4 directions can give fragments that are shaped like octahedrons; crystals commonly cubes | 4 | ||
| Opal | White | Curved, conchoidal fracture; may appear glassy, greasy, resinous, or dull; milky white and bluish-white precious opal shows plays of colors | 5-6 | ||
| D. | Cannot be scratched by an ordinary pocket knife but can be scratched by quartz | ||||
| Feldspar | White | Glassy or pearly luster; good cleavage in 2 directions that meet at an angle of 90° or near 90°; common in granite and pegmatite rocks | 6 | ||
| Opal | White | Curved, conchoidal fracture; may appear glassy, greasy, resinous, or dull; milky white and bluish-white precious opal shows plays of colors | 5-6 | ||
| Quartz | White | Curved conchoidal fracture; occurs as milky quartz, chert, and chalcedony; crystals commonly 6-sided prisms with pyramid-like ends | 7 | ||
| 7. | NONMETALLIC luster, GRAY color | ||||
| A. | Can be scratched by a fingernail | ||||
| Amphibole asbestos | White | Made up of slender, flexible fibers that can be pulled apart | 1-2½ | ||
| Cerargyrite | Shiny white or gray | Appears waxy; knife cuts it smoothly; turns violet brown to black when exposed to light | 1-1½ | ||
| Gypsum | White | Soft; occurs as crystals or as fibrous, granular, compact, or earthy masses | 2 | ||
| Sulfur | White or pale yellow | Will burn with a blue flame; commonly found as crystals, crusts, or grains | 1½-2½ | ||
| Talc | White | Knife cuts it smoothly; feels soapy or greasy; splits into thin flakes | 1 | ||
| B. | Cannot be scratched by a fingernail but can be scratched by a copper penny | ||||
| Amphibole asbestos | White | Made up of slender, flexible fibers that can be pulled apart | 1-2½ | ||
| Anhydrite | White | Commonly occurs as sugary-looking masses | 3-3½ | ||
| Calcite | White | Dilute hydrochloric acid fizzes on calcite; perfect cleavage in 3 directions gives rhomb-shaped fragments | 3 | ||
| Celestite | White | Crystals commonly prism-shaped or flat-looking; some cleavage fragments are flat and slab-like | 3-3½ | ||
| Halite | White | Salty taste; dissolves in water; cube-shaped cleavage fragments | 2½ | ||
| Sulfur | White or pale yellow | Will burn with a blue flame; commonly found as crystals, crusts, or grains | 1½-2½ | ||
| C. | Cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife | ||||
| Anhydrite | White | Commonly occurs as sugary-looking masses | 3-3½ | ||
| Celestite | White | Crystals commonly prism-shaped or flat-looking; some cleavage fragments are flat and slab-like | 3-3½ | ||
| Dolomite | White | Commonly occurs as granular masses and as rhomb-shaped crystals; dilute hydrochloric acid may fizz slightly on dolomite | 3½-4 | ||
| Opal | White | Curved, conchoidal fracture; may appear glassy, greasy, resinous, or dull | 5-6 | ||
| D. | Cannot be scratched by an ordinary pocket knife but can be scratched by quartz | ||||
| Feldspar | White | Glassy or pearly luster; good cleavage in 2 directions that meet at an angle of 90° or near 90°; common in granite and pegmatite rocks | 6 | ||
| Opal | White | Curved, conchoidal fracture; may appear glassy, greasy, resinous, or dull | 5-6 | ||
| Quartz | White | Curved conchoidal fracture; occurs as chert and chalcedony | 7 | ||
| 8. | NONMETALLIC luster, YELLOW color | ||||
| A. | Can be scratched by a fingernail | ||||
| Carnotite | Yellow | Bright canary yellow or lemon yellow; radioactive; occurs as crusts and powdery masses | 2 | ||
| Gypsum | White | Yellowish; soft; occurs as crystals or as fibrous, granular, compact or earthy masses | 2 | ||
| Limonite | Rusty yellowish brown | Brownish-yellow color; may be soft and earthy | 1+ | ||
| Muscovite (white mica) | White | Light colored; splits into thin, flat, transparent sheets that will bend without breaking | 2-2½ | ||
| Sulfur | White or pale yellow | Will burn with a blue flame; commonly found as crystals, crusts, or grains | 1½-2½ | ||
| Uranophane | Light yellow to light yellow orange | Yellow to yellow-orange color; radioactive | 2-3 | ||
| B. | Cannot be scratched by a fingernail but can be scratched by a copper penny | ||||
| Barite | White | Rather heavy; cleavage fragments may be flat and slab-like | 3-3½ | ||
| Calcite | White | Yellowish; dilute hydrochloric acid fizzes on calcite; perfect cleavage in 3 directions gives rhomb-shaped fragments | 3 | ||
| Muscovite (white mica) | White | Light colored; splits into thin, flat, transparent sheets that will bend without breaking | 2-2½ | ||
| Sulfur | White or pale yellow | Will burn with a blue flame; commonly found as crystals, crusts, or grains | 1½-2½ | ||
| Uranophane | Light yellow to light yellow orange | Yellow to yellow-orange color; radioactive | 2-3 | ||
| C. | Cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife | ||||
| Barite | White | Rather heavy; cleavage fragments may be flat and slab-like | 3-3½ | ||
| Opal | White | Curved, conchoidal fracture; may appear glassy, greasy, resinous, or dull | 5-6 | ||
| D. | Cannot be scratched by an ordinary pocket knife but can be scratched by quartz | ||||
| Feldspar | White | Glassy or pearly luster; good cleavage in 2 directions that meet at an angle of 90° or near 90° | 6 | ||
| Garnet | White | Commonly occurs as crystals | 6½-7 | ||
| Opal | White | Curved, conchoidal fracture; may appear glassy, greasy, resinous, or dull | 5-6 | ||
| Quartz | White | Curved conchoidal fracture; brownish-yellow smoky quartz crystals commonly 6-sided prisms with pyramid-like ends; chalcedony and jasper may be a shade of yellow, too | 7 | ||
| 9. | NONMETALLIC luster, BROWN color | ||||
| A. | Can be scratched by a fingernail | ||||
| Gypsum | White | Brownish; soft; occurs as crystals or as fibrous, granular, compact or earthy masses | 2 | ||
| Limonite | Rusty yellowish brown | May be soft and earthy | 1+ | ||
| Muscovite (white mica) | White | Light colored; splits into thin, flat, transparent sheets that will bend without breaking | 2-2½ | ||
| Sulfur | White or pale yellow | Will burn with a blue flame; commonly found as crystals, crusts, or grains | 1½-2½ | ||
| B. | Cannot be scratched by a fingernail but can be scratched by a copper penny | ||||
| Barite | White | Rather heavy; cleavage fragments may be flat and slab-like | 3-3½ | ||
| Biotite (black mica) | White | Dark brown; splits into thin, flat sheets that will bend without breaking | 2½-3 | ||
| Calcite | White | Dilute hydrochloric acid fizzes on calcite; perfect cleavage in 3 directions gives rhomb-shaped fragments | 3 | ||
| Halite | White | Salty taste; dissolves in water; cube-shaped cleavage fragments | 2½ | ||
| Muscovite (white mica) | White | Light colored; splits into thin, flat, transparent sheets that will bend without breaking | 2-2½ | ||
| Sulfur | White or pale yellow | Will burn with a blue flame; commonly found as crystals, crusts, or grains | 1½-2½ | ||
| C. | Cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife | ||||
| Barite | White | Rather heavy; cleavage fragments may be flat and slab-like | 3-3½ | ||
| Dolomite | White | Commonly occurs as granular masses and as rhomb-shaped crystals; dilute hydrochloric acid may fizz slightly on dolomite | 3½-4 | ||
| Fluorite | White | Cleavage in 4 directions can give fragments that are shaped like octahedrons; crystals commonly cube-shaped | 4 | ||
| D. | Cannot be scratched by an ordinary pocket knife but can be scratched by quartz | ||||
| Cassiterite | Pale brown, pale yellow, or white | Brown, reddish brown, or yellowish brown; heavy; dull to brilliant luster | 6-7 | ||
| Feldspar | White | Glassy or pearly luster; good cleavage in 2 directions that meet at an angle of 90° or near 90°; common in granite and pegmatite rocks | 6 | ||
| Garnet | White | Commonly occurs as crystals | 6½-7 | ||
| Quartz | White | Curved conchoidal fracture; brown smoky quartz crystals commonly 6-sided prisms with pyramid-like ends; chalcedony, chert, and jasper may be a shade brown, too | 7 | ||
| Tourmaline | White | Dark brown variety is dravite; notice hardness, striations on crystals, and triangular cross section of some crystals | 7-7½ | ||
| E. | Cannot be scratched by quartz | ||||
| Tourmaline | White | Dark brown variety is dravite; notice hardness, striations on crystals, and triangular cross section of some crystals | 7-7½ | ||
| 10. | NONMETALLIC luster, BLACK color | ||||
| A. | Cannot be scratched by a fingernail but can be scratched by a copper penny | ||||
| Biotite (black mica) | White | Splits into thin, flat sheets that will bend without breaking | 2½-3 | ||
| B. | Cannot be scratched by an ordinary pocket knife but can be scratched by quartz | ||||
| Garnet | White | Commonly occurs as crystals | 6½-7 | ||
| Quartz | White | Curved conchoidal fracture; brownish-black smoky quartz crystals commonly 6-sided prisms with pyramid-like ends; chalcedony and chert may be black, too | 7 | ||
| Tourmaline | White | Black variety is schorl; notice hardness, striations on crystals, and triangular cross section of some crystals | 7-7½ | ||
| C. | Cannot be scratched by quartz | ||||
| Tourmaline | White | Black variety is schorl; notice hardness, striations on crystals, and triangular cross section of some crystals | 7-7½ | ||
| 11. | NONMETALLIC luster, GREEN color | ||||
| A. | Can be scratched by a fingernail | ||||
| Amphibole asbestos | White | Made up of slender, flexible fibers that can be pulled apart | 1-2½ | ||
| Cerargyrite | Shiny white or gray | Light greenish color; appears waxy; knife cuts it smoothly; turns violet brown to black when exposed to light | 1-1½ | ||
| Muscovite (white mica) | White | Light colored; splits into thin, flat, transparent sheets that will bend without breaking | 2-2½ | ||
| Sulfur | White or pale yellow | Greenish; will burn with a blue flame; commonly found as crystals, crusts, or grains | 1½-2½ | ||
| Talc | White | Light greenish color; knife cuts it smoothly; feels soapy or greasy; splits into thin flakes | 1 | ||
| B. | Cannot be scratched by a fingernail but can be scratched by a copper penny | ||||
| Amphibole asbestos | White | Made up of slender, flexible fibers that can be pulled apart | 1-2½ | ||
| Biotite (black mica) | White | Dark green; splits into thin, flat, translucent sheets that will bend without breaking | 2½-3 | ||
| Calcite | White | Dilute hydrochloric acid fizzes on calcite; perfect cleavage in 3 directions gives rhomb-shaped fragments | 3 | ||
| Halite | White | Greenish tint; salty taste; dissolves in water; cube-shaped cleavage fragments | 2½ | ||
| Muscovite (white mica) | White | Light colored; splits into thin, flat, transparent sheets that will bend without breaking | 2-2½ | ||
| Serpentine | White | Two kinds: silky and fibrous, waxy and platy | 2½-4 | ||
| Sulfur | White or pale yellow | Greenish; will burn with a blue flame; commonly found as crystals, crusts, or grains | 1½-2½ | ||
| C. | Cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife | ||||
| Fluorite | White | Cleavage in 4 directions can give fragments shaped like octahedrons; crystals commonly cubes | 4 | ||
| Malachite | Green | Bright green color; dilute hydrochloric acid will fizz on malachite | 3½-4 | ||
| Serpentine | White | Two kinds: silky and fibrous, waxy and platy | 2½-4 | ||
| D. | Cannot be scratched by an ordinary pocket knife but can be scratched by quartz | ||||
| Feldspar | White | Glassy or pearly luster; good cleavage in 2 directions that meet at an angle of 90° or near 90° | 6 | ||
| Garnet | White | Commonly occurs as crystals | 6½-7 | ||
| 12. | NONMETALLIC luster, BLUE color | ||||
| A. | Cannot be scratched by a fingernail but can be scratched by a copper penny | ||||
| Anhydrite | White | Commonly occurs as sugary-looking masses | 3-3½ | ||
| Barite | White | Rather heavy; cleavage fragments may be flat and slab-like | 3-3½ | ||
| Calcite | White | Dilute hydrochloric acid fizzes on calcite; perfect cleavage in 3 directions gives rhomb-shaped fragments | 3 | ||
| Celestite | White | Not quite as heavy as barite; crystals commonly prism-shaped or flat-looking; some cleavage fragments are flat and slab-like | 3-3½ | ||
| Halite | White | Salty taste; dissolves in water; cube-shaped cleavage fragments | 2½ | ||
| B. | Cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife | ||||
| Anhydrite | White | Commonly occurs as sugary-looking masses | 3-3½ | ||
| Azurite | Blue | Bright, intense blue color; dilute hydrochloric acid will fizz on azurite | 3½-4 | ||
| Barite | White | Rather heavy; cleavage fragments may be flat and slab-like | 3-3½ | ||
| Celestite | White | Not quite as heavy as barite; crystals commonly prism-shaped or flat-looking; some cleavage fragments are flat and slab-like | 3-3½ | ||
| Fluorite | White | Cleavage in 4 directions can give fragments that are shaped like octahedrons; crystals commonly cube-shaped | 4 | ||
| Opal | White | Curved, conchoidal fracture; may appear glassy, greasy, resinous, or dull; milky white and bluish-white precious opal shows plays of colors | 5-6 | ||
| C. | Cannot be scratched by an ordinary pocket knife but can be scratched by quartz | ||||
| Feldspar | White | Glassy or pearly luster; good cleavage in 2 directions that meet at an angle of 90° or near 90° | 6 | ||
| Opal | White | Curved, conchoidal fracture; may appear glassy, greasy, resinous, or dull; milky white and bluish-white precious opal shows plays of colors | 5-6 | ||
| Quartz | White | Curved conchoidal fracture; occurs as crystalline quartz and as bluish chalcedony | 7 | ||
| D. | Cannot be scratched by quartz | ||||
| Topaz | White | Perfect basal cleavage gives flat, plate-like fragments; notice hardness | 8 | ||
| 13. | NONMETALLIC luster, RED or PINK color | ||||
| A. | Can be scratched by a fingernail | ||||
| Gypsum | White | Reddish; soft; occurs as crystals or as fibrous, granular, compact, or earthy masses | 2 | ||
| Hematite | Dark reddish brown | Brownish-red color; soft and earthy | 1+ | ||
| Sulfur | White or pale yellow | Reddish; will burn with blue flame; commonly found as crystals, crusts, or grains | 1½-2½ | ||
| B. | Cannot be scratched by a fingernail but can be scratched by a copper penny | ||||
| Anhydrite | White | Pinkish tint; commonly occurs as sugary-looking masses | 3-3½ | ||
| Barite | White | Pinkish tint; rather heavy; cleavage fragments may be flat and slab-like | 3-3½ | ||
| Calcite | White | Pink color; dilute hydrochloric acid fizzes on calcite; perfect cleavage in 3 directions gives rhomb-shaped fragments | 3 | ||
| Cinnabar | Dark red | Dark red or bright yellowish-red color; shiny, brilliant luster when pure; dull and earthy when impure; heavy | 2½ | ||
| Halite | White | Reddish tint; salty taste; dissolves in water; cube-shaped cleavage fragments | 2½ | ||
| Sulfur | White or pale yellow | Reddish; will burn with blue flame; commonly found as crystals, crusts, or grains | 1½-2½ | ||
| C. | Cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife | ||||
| Anhydrite | White | Pinkish tint; commonly occurs as sugary-looking masses | 3-3½ | ||
| Barite | White | Pinkish tint; rather heavy; cleavage fragments may look flat and slab-like | 3-3½ | ||
| Dolomite | White | Pink color; commonly occurs as granular masses and as rhomb-shaped crystals; dilute hydrochloric acid may fizz slightly on dolomite | 3½-4 | ||
| Fluorite | White | Pink color; cleavage in 4 directions can give fragments that are shaped like octahedrons; crystals commonly cubes | 4 | ||
| Opal | White | Reddish color; curved, conchoidal fracture; may appear glassy, greasy, resinous, or dull | 5-6 | ||
| D. | Cannot be scratched by an ordinary pocket knife but can be scratched by quartz | ||||
| Feldspar | White | Glassy or pearly luster; good cleavage in 2 directions that meet at an angle of 90° or near 90° | 6 | ||
| Garnet | White | Commonly occurs as crystals | 6½-7 | ||
| Opal | White | Reddish color; curved, conchoidal fracture; may appear glassy, greasy, resinous, or dull | 5-6 | ||
| Quartz | White | Curved, conchoidal fracture; occurs as rose quartz, as pink chert, and as agate and jasper | 7 | ||
| 14. | NONMETALLIC luster, PURPLE or VIOLET color | ||||
| A. | Cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife | ||||
| Fluorite | White | Cleavage in 4 directions can give fragments that are shaped like octahedrons; crystals commonly cubes | 4 | ||
| B. | Cannot be scratched by an ordinary pocket knife but can be scratched by quartz | ||||
| Quartz, variety: amethyst | White | Curved, conchoidal fracture; amethyst crystals commonly 6-sided prisms with pyramid-like ends | 7 | ||
| 15. | NONMETALLIC luster, COLORLESS | ||||
| A. | Can be scratched by a fingernail | ||||
| Cerargyrite | Shiny white or gray | Appears waxy; knife cuts it smoothly; turns violet brown to black when exposed to light | 1-1½ | ||
| Gypsum | White | Transparent selenite variety commonly occurs as flat, diamond-shaped crystals; splits into thin, flat sheets that will not bend without breaking | 2 | ||
| Muscovite (white mica) | White | Splits into thin, flat, transparent sheets that will bend without breaking | 2-2½ | ||
| B. | Cannot be scratched by a fingernail but can be scratched by a copper penny | ||||
| Barite | White | Rather heavy; cleavage fragments may be flat and slab-like | 3-3½ | ||
| Calcite | White | Dilute hydrochloric acid fizzes on calcite; perfect cleavage in 3 directions gives rhomb-shaped fragments | 3 | ||
| Celestite | White | Not quite as heavy as barite; crystals commonly prism-shaped or flat-looking; some cleavage fragments are flat and slab-like | 3-3½ | ||
| Halite | White | Salty taste; dissolves in water; cube-shaped cleavage fragments | 2½ | ||
| Muscovite (white mica) | White | Splits into thin, flat, transparent sheets that will bend without breaking | 2-2½ | ||
| C. | Cannot be scratched by a copper penny but can be scratched by an ordinary pocket knife | ||||
| Barite | White | Rather heavy; cleavage fragments may be flat and slab-like | 3-3½ | ||
| Celestite | White | Not quite as heavy as barite; crystals commonly prism-shaped or flat-looking; some cleavage fragments are flat and slab-like | 3-3½ | ||
| Dolomite | White | Commonly occurs as granular masses and as rhomb-shaped crystals; dilute hydrochloric acid may fizz slightly on dolomite | 3½-4 | ||
| Fluorite | White | Cleavage in 4 directions can give fragments that are shaped like octahedrons; crystals commonly cubes | 4 | ||
| Opal | White | Curved, conchoidal fracture; transparent hyalite variety resembles ice | 5-6 | ||
| D. | Cannot be scratched by an ordinary pocket knife but can be scratched by quartz | ||||
| Opal | White | Curved, conchoidal fracture; transparent hyalite variety resembles ice | 5-6 | ||
| Quartz | White | Curved, conchoidal fracture; rock crystal quartz commonly 6-sided prism with pyramid-like ends | 7 | ||
| E. | Cannot be scratched by quartz | ||||
| Topaz | White | Perfect basal cleavage gives flat, plate-like fragments; notice hardness | 8 | ||