Geology & formation

Nearly all natural diamonds crystallized in the deep roots of ancient, stable continental blocks called cratons, at pressures above 4 GPa and temperatures of 950–1400 °C. GIA places the top of that stability field at about 140 km, with most gem diamonds growing between roughly 150 and 250 km. Inclusion dating gives them ages above a billion years, some close to 3.5 billion.

Diamonds reach the surface in only three rare magma types: kimberlite, lamproite and lamprophyre. All three form by small amounts of melting deep in the mantle, are rich in volatiles and in magnesium, and erupt fast, at an estimated 4 to 20 m per second. Kimberlite magma can be generated as deep as 200–300 km. It fractures its way upward, gas comes out of solution as pressure drops, and the eruption builds a pipe in three parts: a crater, a diatreme of shattered rock with walls near 82 degrees, and a root zone of intrusive kimberlite. Under Clifford’s rule, diamond-bearing kimberlites erupt through the oldest Archean parts of cratons. Western Australia’s Argyle mine, a lamproite, was a leading producer of pink and brown diamonds until it closed in 2020. Even good ore is lean: USGS puts mined kimberlite grades at 0.1 to 0.6 ppm diamond.

Superdeep diamonds formed at about 300–800 km and make up an estimated 2% of diamonds mined worldwide. They include nitrogen-poor CLIPPIR stones and boron-bearing blue type IIb diamonds.

Erosion frees gems from their host rock, and rivers, waves and currents move them. Gem minerals are hard, chemically resistant and denser than quartz sand, so they collect where currents slow: gravel bars, potholes, old river terraces and beaches. These concentrations are placers; placers laid down by rivers are alluvial. Flawed crystals tend to break up in transport, so placers are often richer in sound stones than their sources. GIA’s review of corundum deposits notes that placers in metamorphic settings, such as Ratnapura and Elahera in Sri Lanka and Ilakaka in Madagascar, yield the highest-quality sapphires.

The largest diamond placer system lies along the Atlantic coast of southern Namibia. The Orange River was the main conduit carrying diamonds from the interior of southern Africa to the coast, and northward longshore drift spread them along beaches and the sea floor for more than 150 km beyond the old river mouth. Gravel size and diamond size both fall off northward. A 2006 study of these deposits put the Namibian coast at more than 55% of the 130 million carats of alluvial diamonds mined in southern Africa up to that point. Marine mining now dominates: a 2020 paper reports a fleet of seven vessels working the sea floor at water depths of up to 150 m and recovering more than three quarters of Namibia’s diamond production.

A pegmatite is an igneous rock made of unusually large interlocking crystals. Most gem-bearing pegmatites are granitic. They crystallize from the last, water-rich fraction of a cooling granite magma, a residue enriched in incompatible elements that do not fit easily into common rock-forming minerals: lithium, beryllium, boron, fluorine, phosphorus, rubidium, niobium, cesium and tantalum. At the low temperatures of this stage, new crystals start more slowly than existing ones grow, so a few crystals get very large instead of many staying small. Recent work suggests growth rates can reach a meter or more per day.

Gem crystals occur most often in open or clay-filled pockets in the core and central zones of the body, where they grew freely into fluid-filled space. Bodies with such cavities are called miarolitic pegmatites, and they sit within or near plutons that intruded to shallow depths in the crust. Pegmatites of the lithium-cesium-tantalum (LCT) family greatly outnumber those of the niobium-yttrium-fluorine (NYF) family, and they are significant producers of gem tourmaline, beryl, spodumene, topaz and garnet.

Well-known pegmatite gem districts include Minas Gerais in Brazil, Madagascar, the Skardu district of Pakistan and San Diego County in California, where the Oceanview mine still yields kunzite. Pegmatites have produced some of the largest crystals known, with spodumene and beryl reported at 10 m or more in length.

Many of the most valued rubies grew in marble, a metamorphic rock formed when limestone recrystallizes under heat and pressure. A belt of marble-hosted deposits runs across Central and Southeast Asia: Jegdalek in Afghanistan, the Hunza Valley and Nangimali in Pakistan, Chumar and Ruyil in Nepal, Mogok and Mong Hsu in Myanmar, and Luc Yen and Quy Chau in Vietnam. All lie in ground deformed by the collision of India with Asia, and radiometric dating gives the Central and Southeast Asian deposits Oligocene to Pliocene ages, about 40 to 5 million years.

GIA’s review of corundum deposits puts ruby growth in these marbles at roughly 620–670 °C and 2.6–3.3 kbar. The crystals sit scattered through the rock with phlogopite, muscovite, scapolite, margarite, spinel, titanite, pyrite and graphite, which is why fine red spinel comes out of the same workings, Mogok above all. The Mogok Stone Tract in Myanmar has produced pigeon’s blood rubies since about 600 CE.

Not every fine ruby deposit is marble-hosted. Montepuez in Mozambique is an amphibolite-type deposit in metamorphosed mafic and ultramafic rocks, and most of its output comes from secondary ground nearby: colluvium of angular fragments, with a smaller share of smooth, rounded alluvial pebbles.

Many blue, green and yellow sapphires are recovered from soils and gravels over young alkali basalt fields. Deposits of this kind run from Tasmania through eastern Australia, Southeast Asia and eastern China to far eastern Russia, and are also worked in Nigeria and Cameroon, in Madagascar and at Aksum in Ethiopia. The basalt did not make the gems. The sapphires are xenocrysts, crystals foreign to the lava that carried them, found in flows and plugs of subalkaline olivine basalt, high-alumina alkali basalt and basanite, in crustal settings stretched above rising mantle plumes. Weathering of those flows leaves the alluvial deposits worked today, mainly in northern New South Wales and central Queensland.

These sapphires have a recognizable chemistry. In a 2007 study, J.-J. Peucat and colleagues showed that magmatic blue sapphires from alkali basalts carry 2000 to 11000 ppm iron, more than 140 ppm gallium and generally under 20 ppm magnesium, giving gallium-to-magnesium ratios above 10. Metamorphic blue sapphires, including the pastel stones from Pailin in Cambodia and material from Mogok, Sri Lanka and Ilakaka, carry under 3000 ppm iron and sit below a ratio of 10. Laboratories still use that split as one line of evidence.

Montana’s Yogo Gulch sapphires are a related magmatic case, but their host is an alkaline lamprophyre rather than basalt.

Emerald needs beryllium, which concentrates in granites and pegmatites, together with chromium or vanadium, which concentrate in dark mafic and ultramafic rocks and in some shales. Most deposits bring the two together: GIA’s review puts the type hosted in metamorphosed mafic and ultramafic rocks at about 70% of world production, with Brazil, Zambia and Russia the main suppliers.

Colombia’s deposits in the Eastern Cordillera formed another way, with no igneous activity involved. The emeralds occur in veins and breccias within Lower Cretaceous black shales. In a 1994 Nature paper, T. L. Ottaway and colleagues showed for the Muzo mine that hydrothermal brines carried evaporitic sulfate to favorable structures, where it was chemically reduced; the sulfur then reacted with organic matter in the shale, releasing the chromium, vanadium and beryllium trapped there. GIA puts these basinal fluids at 300–330 °C and up to 40 wt.% equivalent NaCl, formed at depth where meteoric and formational water met salt beds and evaporite sequences.

The mines fall into two groups. The western zone holds Muzo, Coscuez, Peñas Blancas, Cunas, La Pita and Yacopi; the eastern zone holds the Gachalá, Chivor and Macanal districts.

Australia’s precious opal fields lie in and along the margin of the Great Artesian Basin, whose Cretaceous rocks were laid down in and beside a shallow inland sea, the Eromanga Sea. Coober Pedy and Andamooka in South Australia, White Cliffs and Lightning Ridge in New South Wales and the boulder opal fields of Queensland all sit in these sediments, and opal is recovered mainly from their Cretaceous sandstone and claystone units.

How the opal formed is still argued over. GIA’s survey of the fields sets out a weathering theory, a microbe theory and a syntectonic theory, with more recent work pointing to a redox front moving through a deeply weathered profile. The timing is disputed as well.

Deposit styles differ by field. At Lightning Ridge, black opal occurs as nodules in whitish clay of the Early Cretaceous Finch Claystone; miners call them nobbies, and the deposits are usually less than 30 m below the surface. Much of a nobby is dark gray potch, with precious opal in part of it, and that dark background is what gives fine black opal its play-of-color. In Queensland, opal fills siliceous ironstone concretions that miners call lily pads or nuts, in the Desert Sandstone of Upper Cretaceous age. Opal has also replaced Cretaceous fossils, from microscopic organisms to dinosaur skeletons.

Two materials sit at the edge of gemology: one that is real but almost never large enough to cut, and one whose existence is disputed.

Moissanite is silicon carbide. Natural silicon carbide is very rare and has been found as tiny crystals, usually less than 1.5 mm, in only a few deposits worldwide. The largest known natural crystals come from the Kishon River in northern Israel, where recovered material grew from 0.1 to 1 mm around 2000, to 2.2 mm in 2002, 3.5 mm in 2009 and 4.1 mm in 2012. At those sizes natural crystals cannot be faceted, so the moissanite used in jewelry is laboratory-grown.

Lonsdaleite, also called hexagonal diamond, has been widely used as a marker of asteroid impacts and is thought to play a central part in the change from graphite to diamond. Despite long effort it has never been produced or described as a separate, pure material. In 2014, Péter Németh and colleagues reported in Nature Communications that samples showing the features attributed to lonsdaleite are cubic diamond crowded with {113} twins and {111} stacking faults, including material from the Canyon Diablo meteorite that supplied the original description. They argued the mineral does not exist as a discrete material, and that work resting on it needs rechecking.