Minerals, which often give different rocks their wonderful colors, or make them, sparkle like gems. All rocks are not formed in the same way. One kind of rock, called “sedimentary rock,” has been formed by sediment.
Other rocks were once molten matter deep in the earth. They have been poured out on the surface or forced into cracks in other rocks. Such rocks are called “igneous” rocks, and examples of this type are granite and basalt.
The third kind of rocks was once in some other form (igneous or sedimentary), but has been changed into its present form due to heat or pressure. This type is called “metamorphic” rock, and marble and quartzite are examples of this type of rocks.