A mineral or organic substance used in jewelry.
The only factors that distinguish gemstones from other minerals are their beauty and rarity.
Such rarity is to a large extent what determines their monetary value, diamonds, rubies, sapphires and emeralds being the rarest and therefore most valuable of all.
Gems are not always minerals: organic substances such as pearls, amber and coral are also classified as gems.
Certain gems such as apatite, diopside, hematite, pyrite and tourmaline are also used in homeopathic medicines.