Most rocks are made of various types of minerals, but some are made of animal shells or compressed plant parts. Minerals are natural inorganic solids with a specific chemical composition. Crystals are expressions of the atomic structures of minerals.
Rocks are mainly divided into the categories igneous, sedimentary and metamorphic, depending on how they are formed. Minerals differ from each other based on their chemical and physical properties. Geologists identify minerals by their features, including color, streak, crystal pattern, hardness, luster and specific gravity. The liquid rocks inside the earth cool and harden to form crystals over a period of time. Amethyst is a common quartz crystal.