Gem

Gem, or gemstone, is a mineral or organic material valued for its beauty, rarity, and durability and used chiefly in jewelry and ornamentation. Most gems are crystalline substances, though some organic materials such as pearls, coral, and amber are also considered gems. The term can also apply to lab-grown crystals that imitate natural stones.

Constituent varieties include diamond, ruby, sapphire, emerald, topaz, opal, and garnet, along with quartz varieties and many other minerals. Properties that influence value include color, clarity, cut, and carat weight. Hardness (Mohs scale), refractive index, dispersion, and pleochroism affect wear and appearance.

Gems form through geological processes in the Earth's crust or mantle, and some occur in hydrothermal veins. Natural stones are mined from a range of environments; popular sources include Myanmar for rubies, Sri Lanka for sapphires, Colombia for emeralds, and Australia for opals. Treatments such as heat, irradiation, diffusion, or fracture-filling are commonly applied to enhance color or clarity.

Synthetic or lab-grown gems are produced to imitate natural stones, using methods such as hydrothermal growth or high-pressure high-temperature processes; synthetic diamonds can be created by HPHT or CVD. Some stones use simulants that resemble real gems but have different chemical composition.

Gems are graded by several laboratories, using criteria beyond the four Cs, including origin and treatment status. Market value depends on scarcity, demand, and reputation of the gem variety, with notable gem types making up most fine jewelry and industrial applications that rely on diamond's hardness.