John Allen Spector, age 9, of Allentown, Pa., for his question:

'That are the three main rock types?

There are hundreds of rocky minerals, and a rockhound needs a way to sort them into groups. tae could sort them by their colors. But this does not tell us much about them. The most important thing we want to know is how the different rocks were formed. The three main types are called sedimentary rocks, igneous rocks and metamorphic rocks.

Sedimentary rocks, such as limestone, were deposited under water. They began as dregs and sediments that sank to the bottom. Igneous rocks, such as granite, were formed by fire. They began deep underground, when fiery volcanos melted mixtures of minerals and steamy gases. This hot lava erupted to the surface, where it cooled and formed hard solid rocks. Later, some of the sedimentary and igneous rocks were heated and crushed in the earth's crust. They were changed completely and became metamorphic rocks. Through the ages, some sedimentary limestones became metamorphic marbles. Some of the tough igneous granites became tougher gneiss, which is a metamorphic type rock.