Tourmaline crystals, growing amid feldspar crystals in a cavity in granite, from Paris, Me.

Gold in quartz, from California

Native silver in calcite

Argentite, the black masses throughout the white quartz

Pyrargyrite as it appears after moderate exposure to the light.

Crystal form of Pyrargyrite

Prousite as it appears after moderate exposure to the light

Native copper from Michigan

Chalcopyrite in tetrahedrons and an occasional octahedron.

Chalcocite crystals with the bluish tarnish

Tetrahedrite crystals

Cuprite, the red crystals showing characteristic color, other showing the green tarnish of malachite

Malachite (green) and azurite (blue), the two minerals shown together as they very commonly occur

Limonite

The crystal form in which goethite is found, p is the prism faces, b and c are faces formed by beveling the edges of the prism, o is the pyramidal face characteristic of the ends

Hematite, Clinton iron ore, oolitic

Siderite crystals

Pyrite crystals

Marcasite in concretionary form with radiate structure

Galena in crystals

Pyromorphite crystals (green)

Sphalerite, some the normal yellow and some crystals with the reddish tinge. (White is dolomite)

Zincite

Smithsonite in yellow crystals

Franklinite in octahedral crystals

Arsenopyrite, showing crystals massed so as to be incompletely developed

Realgar as it usually occurs in powdery incrustations

Large crystal of stibnite, the light colored face is the one parallel to which cleavage occurs

Niccolite as a vein in slate

Cobaltite, silver color, with pink tinge

Smaltite, pink is cobalt bloom

Carnotite from southwest Colorado

Cinnabar

Amethyst, not however deep enough colored for gems

Jasper, with botryoidal surface

Banded Agate from Brazil

Common Opal from Arizona

Siliceous sinter or Geyserite from The Yellowstone Park

A group of Microcline crystals from Pike’s Peak, Colo.

Labradorite, showing multiple twinning (the striation), and the iridescent play of colors

Crystal form of a pyroxene; a and b prism faces, m the beveled edge between two prism faces

Cross section of a pyroxene crystal showing the lines of intersection of the two cleavage planes

Cross sections of pyroxenes, showing typical forms taken by crystals

Augite crystals, in crystalline limestone

The dodecahedron and the 24-sided figure characteristic of garnets

The garnet, grossularite

The garnet alamandite

Beryl of gem quality

Zircon in syenite

Cyanite crystals in schist

A crystal of mica, showing basal cleavage

Crystal form typical of topaz

A topaz crystal from Brazil

Crystal form typical of staurolite when simple

A typical twin of staurolite

Serpentine

Chlorite

Apatite crystals in crystalline calcite

The ends of apatite crystals showing common modes of termination

A group of fluorite crystals

A group of halite crystals

Amber

Two bottles of petroleum, the left hand one with a paraffin base, the right hand one with an asphalt base

Mica schist, with garnets

Chlorite schist

Serpentine, composed of serpentine, hematite, and some calcite

Cube

Octahedron

Dodecahedron

A square prism

Octahedron

A Rectangular prism

Octahedron

The rectangular prism askew

The octahedron

A cross section of the prism with its edges beveled so that the b faces are obliterated by the m faces, and a six-sided prism is formed (pseudo-hexagonal)

Basal form of the triclinic system

The six-sided prism

The double pyramid

The rhombohedron

Tetrahedrons showing characteristic manner in which tetrahedrite occurs

A cube with the edges beveled and the corners cut in a form characteristic of cuprite

Two intergrowing or twinned quartz crystals

Diagram of the typical quartz crystal, p prism faces, l left hand rhombohedron, r right hand rhombohedron

A quartz crystal on which the left hand rhombohedron is represented by small faces while the right hand rhombohedron has large faces

Crystal forms of hematite, A the rhombohedron with the edges beveled; B the tabular form, resulting from the excessive development of the two o faces opposite each other

A typical crystal of magnetite

The rhombohedron typical of siderite

The pyritohedron

The pyritohedron with certain of its edges beveled by the cube faces, to show the relationship of these two forms

The pyramid, n the prism face, m the beveled prism, p the octahedral face, and o the edge of the octahedral faces beveled

The simple type of twinning

A multiple twin where three crystals grow through each other

Forms in which anglesite occurs: l the pyramid face, p the prism face, o the vertical edge of the prism beveled, m the horizontal edge of the prism beveled, n a further beveling of the horizontal edge of the prism. D the tabular, E the prismatic form

A characteristic form in which sphalerite may occur; being the combination of, d the dodecahedron, o the octahedron, and t, a 24-sided figure

Characteristic form for zincite crystals, n the hexagonal prism, and p pyramidal faces on it

Typical form of crystal of willemite: p the prism, r rhombohedron faces on end, ½ r a second lower rhombohedron

Moss agates, showing the dendritic growth of manganitic minerals, like manganite or pyrolusite

Moss agates

Crystal form of manganite

Crystals of green corundum in syenite, from Montana

Typical crystal forms of corundum: A the elongated prism with the alternate corners cut by rhombohedral faces, B the tabular prism, C the double pyramid

Cassiterite, twinned crystals

The crystal form in which both cassiterite and rutile occur when in simple crystals, p prism faces, m beveling of the prism, o octahedral face, n beveling of the edge between octahedral faces

Multiple twinning characteristic of rutile

Crystal of Spinel

A the cleavage form, rhombohedron with the faces curved

B the rhombohedron with the corners cut, as it often occurs

C the form found in gypsum or anhydrite

Orthoclase, a cleavage piece, a and b the perfect cleavage planes, and c the imperfect cleavage plane

A the simple crystal

B the twinned form

C the twinned form in which the crystals are intergrowing

Diagram of a multiple twin of a plagioclase feldspar

A a typical crystal

B cross section showing the intersection of cleavage planes

C and D cross sections to show variations in outline

Tremolite in silky fibrous crystals. Asbestos

Hornblende crystals in quartzite

Epidote crystals

Typical forms of epidote crystals; p prism faces, m, n, x, and y beveled edges of the prism, o octahedral faces

A side view; B and C ends to show terminations; p prism faces, m beveling of prism edges, r a low rhombohedron on the end, s the opposite rhombohedron, b basal face, and the other faces represent bevelings

A group of barite crystals

Outline of the typical tabular barite crystal

The six-sided double pyramid, composed of three interpenetrating crystals, typical of witherite and strontianite

The typical form of analcite

A typical natrolite crystal

The typical crystal form of stilbite

A sheaf-like bundle of fibrous crystals, typical of stilbite

A group of calcite crystals

A the rhombohedron formed by cleavage

B a rhombohedral crystal truncated by the basal plane

C the scalenohedron

D the scalenohedron truncated by the rhombohedron

E the scalenohedron on a prism

A the simple crystal

B a needle-like form, twinned

C cross section to show how the form may appear six-sided

Typical form of the anhydrite crystal

A piece of gypsum looking on the surface of the perfect cleavage, and showing the two other cleavages as lines, intersecting at 66°. Twinning is also shown

A simple crystal of gypsum

Twin crystals of gypsum

Sulphur crystals

Ice crystals, the top one, the end of a hexagonal prism; the two lower figures multiple twins as in snow flakes

The Devil’s Tower, Wyoming, an example of igneous rock with columnar structure, and resting on sedimentary rocks. Courtesy of the U. S. Geological Survey

A coarse granite

Graphic granite

Syenite

Gabbro

Basalt-porphyry. The large white crystals are phenocrysts of plagioclase feldspar

Basalt-obsidian

Amgydoloid

The north face of Scott’s Bluff, Neb., showing sedimentary sandstones above and clays below. The type of erosion is characteristic of arid regions. Courtesy of the U. S. Geological Survey

Breccia

Conglomerate

Calcareous shale

Coquina

Foramenifera from Chalk; enlarged about 25 diameters

Encrinal Limestone; fragments of the stems, arms and body of Crinoids

A diatomaceous earth magnified 50 times

B and C two diatoms from the above enlarged 250 times. After Gravelle, by the courtesy of Natural History

A metamorphic rock, showing the contortion of layers due to expansion under heat

A conglomerate partly metamorphosed to a gneiss. Note the flattened pebbles and the alternation of the intermediate material to mica scales, etc.

A typical gneiss

Phyllite

A white marble, with black streaks due to graphite

Claystones, simple and compound

A line concretion, which on splitting disclosed a fern leaf of the age of the coal measures

A septeria from Seneca Lake, N. Y.

Pisolite

A geode filled with quartz crystals

A quartz pebble from the bed of a New England brook

A pebble of schist and granite from the foot of Mt. Toby, Mass.

An iron-nickel meteorite, of 23 lbs. which fell in Claiborne Co., Tenn.

An etched slice of an iron meteorite which fell in Reed City, Osceola Co., Mich.

A stony meteorite, about natural size, which fell in 1875, in Iowa Co., Iowa