Minerals are solid, inorganic materials with specific chemical compositions and distinct internal structures. When minerals form inside the earth, their atoms lock together to create crystals. Under the right conditions and given sufficient space, crystals can "grow" into regular geometric forms with smooth surfaces. Depending upon their atomic structure and pattern of growth, they can acquire a wide range of spectacular shapes.

The atomic arrangement of a mineral crystal depends not only upon its chemical composition but also upon external forces For example, pure carbon commonly crystallizes as graphite, one of the softest minerals and the component of pencil leads. But tremendous pressure and heat can transform carbon's crystal structure so that it forms diamonds, the hardest of all minerals. Thus some minerals can betray the circumstances of their own origins and allow us to discover more about the history of our planet and the forces that shaped it.

Gems are cut and polished mineral crystals prized for their beauty, durability, and rarity. We commonly stretch the definition of gem to include some beautiful and durable organic substances, like pearls, amber, jet, and coral. What determines the value of a gem is somewhat subjective, since our perceptions of what is beautiful and precious change over time.