Why are there so many different kinds of rocks?

Is there any boy anywhere in the world who hasn't filled up his pockets at some time with all kinds of strange rocks? The different sizes and shapes, some rough and smooth, the different colors, some almost like precious gems, just make us want to collect them!

What makes rock look so fascinating is the fact that they are composed of one or more minerals. It is these minerals which often give different rocks their wonderful colors, or make them sparkle like gems.

All rocks aren't formed in the same way. One kind of rock, called "sedimentary rock", has been formed by sediment. This means that substances are laid down long ago by water, wind, ice, or by the work of plants and animals. Since they are laid down in layers which are called "strata", these rocks are called "stratified" rocks. The tiny particles of which such rocks are made are usually rounded, since the sharp edges and corners have been worn off while the particles were being rolled along stream beds, washed by the waves, or blown by winds. Sandstones and limestones are typical sedimentary rocks.

Other rocks were once molten matter deep in the earth. They have been poured out on the surface or forced into cracks in the other rocks. Such rocks are called "igneous" rocks, and examples of this type are granite and basalt.

aThe third kind of rocks was once in some other form (igneous or sedimentary), but has been changed into its present form by heat or pressure. This type is called "metamorphic" rock, and marble and quartzite are examples of this type.

In some rocks, different metal-bearing minerals are found mixed with the other rock substances. If there is enough of the metal to make it worth while separating it, the rock containing it is called an "ore".