Igneous rocks form when molten rock cools and crystallizes. They supply some of the world's most important gems in three main ways: pegmatites grow large crystals of tourmaline, beryl and topaz; basalt lavas carry sapphire, zircon and peridot to the surface; and kimberlite and lamproite eruptions bring diamonds up from the mantle. Each setting depends on how the magma formed, what it carried and how fast it cooled.
Magma and how it crystallizes
Magma forms where rock in the crust or upper mantle partly melts. That happens when temperature rises, when pressure drops as rock rises, or when water lowers the melting point, as it does above subduction zones. Magma is lighter than the rock around it, so it rises. Some of it erupts as lava, and some stalls and cools underground.
As magma cools, minerals crystallize in a broad order. High-temperature minerals such as olivine and pyroxene tend to form first, and quartz, potassium feldspar and mica form last. Elements that do not fit easily into the common rock-forming minerals, such as lithium, beryllium, boron and fluorine, stay in the remaining melt and become more concentrated. That leftover fluid is where many gem minerals come from.
Intrusive and extrusive rocks
Geologists divide igneous rocks by where they cool.
Intrusive (plutonic) rocks cool slowly underground, which gives crystals time to grow. Granite, diorite and gabbro are coarse-grained intrusive rocks with crystals you can see by eye.
Extrusive (volcanic) rocks cool quickly at or near the surface, so their crystals are small or absent. Basalt, andesite and rhyolite are fine-grained. Obsidian is volcanic glass that cooled too fast for crystals to form at all.
Rock composition also matters. Felsic rocks such as granite are rich in silica, feldspar and quartz. Mafic rocks such as basalt and gabbro are richer in iron and magnesium. Ultramafic rocks such as peridotite, made mostly of olivine, come from the mantle.
| Igneous setting | Typical rock | Gems |
|---|---|---|
| Granite pegmatite | Coarse quartz, feldspar, mica | Tourmaline, beryl (aquamarine, morganite, heliodor), topaz, spodumene (kunzite), garnet |
| Alkali basalt | Fine-grained dark lava | Sapphire, zircon, peridot, some ruby, garnet |
| Mantle xenoliths | Peridotite carried up in lava | Peridot, pyrope garnet, chrome diopside |
| Kimberlite and lamproite | Volcanic pipes from deep mantle sources | Diamond, pyrope garnet |
| Rhyolite and volcanic cavities | Silica-rich lava | Red beryl, topaz, precious and fire opal |
| Basalt gas cavities | Vesicles in lava flows | Amethyst and agate geodes |
Pegmatites
Pegmatites are igneous rocks with very large crystals, from a few centimeters to several meters long. Most form at the end of granite crystallization, when the remaining melt is rich in water and rare elements. Water lowers the viscosity of the melt, so atoms move freely and crystals grow large. Open spaces called pockets or miarolitic cavities let crystals grow with full faces.
Pegmatites are the main source of these gems:
- Tourmaline, including multicolor and watermelon crystals, from boron in the melt.
- Beryl in its non-emerald varieties: aquamarine, morganite and golden heliodor.
- Topaz, from fluorine-rich fluids.
- Spodumene, including pink kunzite, from lithium.
- Spessartine garnet, apatite and some chrysoberyl.
Gem pegmatites are found in Brazil's Minas Gerais, Madagascar, the Hindu Kush of Afghanistan and Pakistan, Nigeria, Mozambique and southern California's San Diego County, among other places. The gems of Minas Gerais page covers one of the richest pegmatite districts.
Basalt-hosted sapphire, zircon and ruby
Many of the world's sapphire fields sit on or near alkali basalts. These include fields in eastern Australia, Thailand's Chanthaburi area, Cambodia's Pailin and Nigeria. The sapphires and zircons did not crystallize from the basalt itself. Geologists consider them xenocrysts, crystals that formed earlier at depth and were picked up and carried to the surface by the rising lava. Basalt-related sapphires are often dark blue to green and rich in iron. The rubies of the Thai-Cambodian border formed in a similar way.
Because basalt weathers quickly in tropical climates, these gems are usually mined from alluvial gravels and weathered soil next to the volcanic fields, not from the hard rock.
Peridot from the mantle
Peridot is gem-quality olivine, the main mineral of the upper mantle. It reaches the surface in two main ways. Basalt lava can carry chunks of mantle peridotite (called xenoliths) to the surface, which is how the peridot at San Carlos, Arizona formed. Peridot also occurs in slices of mantle rock pushed up onto continents during mountain building, as on Zabargad island in the Red Sea and in Pakistan. The peridot page covers these sources.
Kimberlite, lamproite and diamonds
Diamonds form roughly 150 to 200 kilometers down, beneath old, stable parts of continents called cratons. There, carbon crystallizes as diamond because of the high pressure. The diamonds sit in the mantle until a rare, gas-rich magma called kimberlite rises fast enough to carry them up. Kimberlite eruptions punch carrot-shaped pipes through the crust. If the magma rose slowly, the diamonds would turn to graphite or burn away, so the speed of these eruptions is part of why diamonds survive.
Kimberlite pipes host the major diamond mines of South Africa, Botswana, Russia and Canada. Lamproite, a related volcanic rock, hosted Australia's Argyle mine, which closed in 2020 and was known for pink diamonds. Diamond geology covers indicator minerals and why most pipes contain few or no diamonds, and diamond mining covers how they are worked.
Diamonds themselves usually formed long before the kimberlite that carried them. Many diamonds are one to three billion years old, while their host pipes are often much younger.
Volcanic rocks and gas cavities
Some gems form in the cooled lava after eruption. Gas bubbles trapped in basalt flows leave hollow cavities. Later, silica-rich groundwater fills them with layers of agate or lines them with amethyst crystals. Strictly speaking, the gems form from low-temperature fluids, but the host is igneous. The large amethyst geodes of Rio Grande do Sul in Brazil and Artigas in Uruguay formed this way.
In silica-rich rhyolite, gas cavities and fractures host topaz, red beryl and fire opal. Red beryl, one of the rarest gems, comes from rhyolite in Utah's Wah Wah Mountains.
Weathering and the move to placers
Igneous rocks do not stay in place forever. When they weather, durable gems such as sapphire, zircon, diamond and spinel survive and wash into rivers, joining the sedimentary part of the rock cycle. Much of the world's sapphire and historic diamond supply has come from these alluvial deposits instead of the igneous rock itself.
Frequently asked questions
What is a pegmatite?
A pegmatite is a very coarse-grained igneous rock, usually related to granite, that formed from water-rich leftover melt. It is the main source of gem tourmaline, aquamarine, topaz and kunzite.
Do sapphires form in basalt?
Sapphires found with basalt usually formed earlier at depth. The basalt picked them up and carried them to the surface, and they are mostly mined from gravels and soil near the lava fields.
Why are diamonds found in kimberlite?
Kimberlite magma rises fast from deep in the mantle and carries diamonds with it. The rapid ascent keeps them from turning into graphite.
Is obsidian a gemstone?
Obsidian is natural volcanic glass. It is used for carvings, beads and cabochons, though it is soft and has no crystal structure.