Many minerals form beautiful crystals, but the most prized of all are gemstones. Uncut gems often look fairly ordinary like rocks. It's only when they are cut and polished that they obtain the brilliance and lustre that gives them their value.
Historically, gems have been divided into precious and semiprecious classes. There are a number of semiprecious gems, many quite beautiful, but diamonds, rubies, sapphires, and emeralds continue to qualify as "precious."
More than 4,000 naturally occurring minerals inorganic solids that have a characteristic chemical composition and specific crystal structure have been found on Earth. They are formed of simple molecules or individual elements arranged in repeating chains, sheets, or three-dimensional arrays.
Minerals are typically formed when molten rock, or magma, cools, or by separating out of the mineral-rich water, such as that in underground caverns. In general, mineral particles are small, having formed within confined areas such as lava flows or between grains of sediments. Large crystals found in geodes and other rocks are relatively rare.