Howstuffworks "Petrology"

Andesite

Andesite, a fine-grained rock that is widely distributed in the earth's crust.

Basalt

Basalt, one of the most abundant rocks in the earth's crust. It is an igneous rock (one formed by the cooling of molten materials).

Bentonite

Bentonite, the name given to a group of industrially useful clays containing the mineral montmorillonite.

Chalk

Chalk, a very soft, powdery variety of limestone. Like all limestone, it consists mainly of calcium carbonate.

Clay

Clay, an earthy material composed of very small particles of weathered rock. The particles are less than 0.00015 inch (0.004 mm) in diameter, much smaller than sand particles.

Conglomerate (geology)

Conglomerate, in geology, a sedimentary rock composed of rounded rock fragments cemented together by a finer-grained material such as sand or clay.

Diabase

Diabase, an igneous rock similar to basalt in composition, but with coarser grains.

Diatomite

Diatomite, also called Diatomaceous Earth and Kieselguhr, a substance composed primarily of fossilized diatom shells.

Fire Clay

Fire Clay, a variety of clay that can withstand intense heat. In general, it is any clay that can withstand temperatures of 2,900 F.

Fuller''s Earth

Fuller's Earth, an earthy substance resembling potter's clay, but lacking its putty-like quality.

Gneiss

Gneiss, a coarse-grained metamorphic rock similar in composition to granite.

Granite

Granite, a coarse-grained rock widely distributed in the earth's crust. Granite is very hard and takes a beautiful polish.

Gravel

Gravel, small stones found in natural deposits or beds. The stones normally range in size from 14 inch (6 mm) to 2 12 inches (6 cm) in diameter, although the range sometimes is much larger.

Limestone, a sedimentary rock that occurs in many areas of the world. Limestone consists chiefly of calcite (calcium carbonate), but it usually contains numerous other minerals, including dolomite, flint, alumina, bitumen, and pyrites.

Marble

Marble, a snowy white or richly colored ornamental stone. Polishing brings out the handsome grain, network of delicate veins, and cloudy hues that give marble its special beauty.

Marl

Marl, a natural mixture of clay and calcium carbonate. It is a loose, earthy substance, usually gray or bluish in color.

Obsidian

Obsidian, a natural form of glass. It is formed when lava, brought to the surface of the earth by volcanoes, cools so rapidly that crystals have no time to form.