Folks in Montana usually expect snow or rain to fall from the sky, not ash. But the 1980 eruption of Mount St. Helens in Washington flung ash across state lines. What is this gritty, gray stuff? See more »

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. See more »

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

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. See more »

Rock, the solid material that makes up the earth's crust. Soil, sand, and other loose natural materials are not usually considered to be rock, although they are derived principally from rocks. See more »

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

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). See more »

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

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

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