(c) Anthony K. Grafton 2003

Between a Rock and a Hard Mineral

Did you know that the official Kentucky State Mineral, coal, is not a mineral at all? Coal is a rock. Geologists define rock and minerals quite differently.

Minerals are naturally occurring solids with a definite chemical composition. This means that any two samples of the same pure mineral will contain the same elements in the same proportions. There are over two thousand known minerals, but fortunately only a few dozen are common. Quartz, hematite, halite (salt), gypsum, and even ice are good examples of minerals. Because the chemical makeup of minerals is not easy to determine in the field, we often identify minerals by easy-to-see physical properties such as shape, color, luster, and hardness.

Rocks, on the other hand, are solids usually made up of more than one type of mineral. (Technically, a rock can also be made of only one mineral: diamond, gold, silver, and sulfur are examples of minerals that occur naturally by themselves.) Two rocks of the same kind might contain different proportions of the minerals that make them up. Granite, marble, shale, and sandstone are common rock types. Rocks can be classified as igneous (made from cooling lava), sedimentary (made up of tiny pieces of rock that have been geologically combined together), or metamorphic (sedimentary rock that has been transformed by great heat and pressure).

Coal is a rock and not a mineral because it does not have a definite chemical composition. It is made up of bits of ancient compressed plant material, and different samples of coal might have very different properties. Coal has been economically very important to Kentucky over the years. Do you know the names of the different kinds of coal?