Gemstones come from deep inside the Earth, from its surface waters, from living organisms, and in rare cases, from outer space. Most form through geological processes involving extreme heat, pressure, or mineral-rich fluids over millions to billions of years. The oldest known gemstone mineral, zircon, dates back more than 4.4 billion years, making it older than any rock on Earth’s surface today.
Deep Earth: Where Diamonds Begin
Diamonds form deeper than any other common gemstone. Most crystallize at depths between 150 and 700 kilometers below the surface, where temperatures exceed 1,000°C and pressures are immense. At these depths, carbon atoms are squeezed into the tightly packed crystal structure that gives diamonds their hardness. Some diamonds originate even deeper, in the lower mantle, where carbon-bearing minerals react under pressures above 80 billion pascals and temperatures reaching 3,100 K.
These crystals reach the surface through violent volcanic eruptions that create narrow, carrot-shaped pipes of a rock called kimberlite. The eruptions move fast enough to carry diamonds upward before the drop in pressure can convert them back to graphite. The kimberlite pipes found in southern Africa, Russia, and Canada are the primary sources of mined diamonds today.
Cooling Magma and Igneous Rocks
When molten rock cools, minerals crystallize in a predictable sequence based on temperature. Gemstones that form this way include sapphires, rubies, and certain garnets. The specific minerals that appear depend on the chemistry of the magma and how slowly it cools. Slower cooling generally produces larger, better-formed crystals.
A special type of igneous rock called pegmatite is responsible for an unusually wide range of gemstones. Pegmatites form from the last remnants of cooling magma, which are enriched in water and rare elements like beryllium, lithium, and boron. This concentrated cocktail produces tourmaline, topaz, aquamarine, and many other collector favorites. The crystals in pegmatites can grow exceptionally large because the fluid-rich environment allows atoms to migrate easily into growing crystal faces.
Hot Fluids and Hydrothermal Veins
Some of the world’s most prized gemstones form not from magma itself but from superheated water circulating through cracks in rock. Colombian emeralds, widely considered the finest in the world, crystallize in veins running through dark shales and limestones at temperatures of roughly 300 to 350°C. Hot, mineral-laden fluids from surrounding sedimentary basins carry the rare elements beryllium, vanadium, and chromium into fractures, where a sudden drop in pressure triggers rapid crystal growth.
This hydrothermal process also creates opals and turquoise, though at much lower temperatures. Opal forms when silica-rich water seeps into voids in rock and gradually deposits microscopic spheres of silica. The precise stacking of those spheres is what produces opal’s signature play of color. Turquoise precipitates from copper-bearing groundwater in arid environments, which is why it’s commonly found in the American Southwest, Iran, and parts of China.
Heat and Pressure Transform Existing Rock
Metamorphic gemstones form when existing rocks are subjected to intense heat, pressure, or both, without fully melting. This process rearranges atoms into new mineral structures. Rubies and sapphires frequently form this way, as do tanzanite, lapis lazuli, and jade. The deep blue of tanzanite, for instance, comes from trace amounts of vanadium incorporated into the crystal during metamorphism of existing sedimentary rock in Tanzania.
Garnets are classic metamorphic gems, often growing as rounded crystals in rocks that have been buried and heated during mountain-building events. The specific color of a garnet depends on which metals are present: iron produces reds and oranges, chromium yields greens, and manganese creates pinks and purples.
Rivers and Riverbeds: Secondary Deposits
Many gemstones are not mined where they originally formed. Over millions of years, wind, rain, and flowing water break down the rocks containing gems. Hard, durable stones like diamonds and sapphires survive this weathering process because they resist abrasion. Rivers carry them downstream and deposit them in gravel beds, beaches, and floodplains, creating what geologists call alluvial or placer deposits.
This is why some of the richest gem-producing regions in the world, like Sri Lanka and Myanmar, yield an extraordinary variety of stones from riverbeds and shallow pits rather than deep mines. A single alluvial deposit can contain sapphires, rubies, garnets, spinels, and zircons all mixed together, because the river has collected them from multiple source rocks upstream.
Gems From Living Things
Not all gemstones are minerals. A handful come from biological processes, and they’re among the oldest materials used in jewelry.
Pearls are the only gems created inside a living animal. When a foreign particle enters an oyster or mussel, the mollusk coats it in layer after layer of nacre, the same iridescent material that lines the inside of the shell. Over several years, those layers build up into a pearl. Cultured pearls follow the same biological process, but a technician deliberately introduces the irritant.
Amber is fossilized tree resin. Millions of years ago, sticky resin oozed from the bark of ancient trees and hardened over time through chemical reactions that transformed it from a soft, organic goo into a durable, translucent gem. Baltic amber from Russia, Lithuania, and Estonia is among the most well known, and pieces occasionally contain perfectly preserved insects or plant material tens of millions of years old. Jet, a lesser-known organic gem, is fossilized wood from prehistoric trees compressed under extreme pressure in waterlogged or marine environments.
Gems From Outer Space
A small number of gemstones have arrived on Earth inside meteorites. The best-documented example is peridot, the green variety of the mineral olivine. A class of meteorites called pallasites contains clusters of yellowish-green olivine crystals embedded in a nickel-iron matrix. These pallasites are thought to come from the boundary between the core and mantle of an ancient, shattered planetesimal.
Gem-quality extraterrestrial peridot has been recovered from meteorites found in Argentina, Kentucky, Siberia, New Mexico, Australia, and Kansas. Compared to Earth-sourced peridot, the space-born stones tend to be less vivid in color and sometimes appear slightly muddy. They also contain tiny needle-like inclusions never observed in terrestrial peridot, along with microscopic particles of metallic nickel-iron, a direct fingerprint of their meteoritic origin.
Where the World’s Gemstones Are Mined
Gemstone mining happens on every continent except Antarctica, but production concentrates in a handful of countries. For diamonds, Russia leads global output at an estimated 21 million carats per year, followed by Botswana (18 million), Canada (16 million), Angola (8.8 million), and South Africa and Namibia (each around 2.4 million).
Colored gemstones are more geographically scattered. Colombia dominates the emerald market. Myanmar, Sri Lanka, and Madagascar are major sources of rubies and sapphires. Tanzania produces tanzanite exclusively, since it occurs in only one location on Earth. Brazil yields an enormous variety, including tourmaline, topaz, and aquamarine from its pegmatite-rich geology. Australia is the world’s leading source of opal.
Mining methods range dramatically. Large-scale operations use open-pit mines that can stretch kilometers wide and plunge more than 150 meters deep. At the other extreme, artisanal miners in countries like Sri Lanka, Madagascar, and parts of Africa work with shovels, sieves, and pans, often digging by hand in riverbeds to recover alluvial stones.
Lab-Grown Gemstones
Humans have learned to replicate the conditions that produce gemstones naturally. Lab-grown diamonds are the most prominent example, and two methods dominate production.
The first, called high-pressure high-temperature (HPHT), mimics what happens deep underground. A small diamond seed is placed in a press, surrounded by a high-purity carbon source and a metallic mixture of iron, nickel, or cobalt. The chamber is heated to 1,300 to 1,600°C at pressures above 870,000 pounds per square inch. The metal melts and dissolves the carbon, which then migrates to the seed crystal and builds up atom by atom into a full diamond.
The second method, chemical vapor deposition (CVD), works completely differently. Diamond seeds are placed in a vacuum chamber filled with a carbon-containing gas, typically methane. A microwave beam breaks the gas molecules apart, and individual carbon atoms drift down and deposit onto the seed crystal at temperatures of 900 to 1,200°C. The result is chemically identical to a mined diamond, though the growth patterns and trace impurities differ in ways gemologists can detect.
Lab-grown versions of rubies, sapphires, and emeralds have been commercially available for decades, produced through various methods that dissolve and recrystallize the same minerals found in nature. These stones share the same chemical composition and crystal structure as their natural counterparts, with the primary differences being their origin story and their price.