Gemstones crystallize through igneous, metamorphic, hydrothermal, or sedimentary processes under specific combinations of temperature, pressure, and chemistry. Igneous gems like diamond form deep in the Earth's mantle at pressures exceeding 45 kilobars and temperatures above 900 °C, then reach the surface via kimberlite pipes.
Metamorphic gems such as ruby and sapphire develop when existing rocks recrystallize under heat and pressure in tectonic collision zones. Hydrothermal gems, including many emeralds and quartz varieties, precipitate from hot, mineral-rich fluids circulating through fractures. Sedimentary processes concentrate durable gems in alluvial (placer) deposits after erosion and water transport. Each process leaves diagnostic signatures in crystal habit, inclusions, and trace elements.