Every gemstone formed somewhere in the rock cycle, the slow loop in which rocks melt, cool, wear down, get buried and transform again. Geologists sort rocks into three families: igneous, sedimentary and metamorphic. Each family has its own gems, and many deposits form where one stage of the cycle meets another.
How the rock cycle works
Igneous rock forms when molten rock (magma underground, lava at the surface) cools and crystallizes. Once exposed, any rock can be broken down by weathering and erosion. Rivers, wind and ice carry the pieces away and lay them down as sediment, which hardens into sedimentary rock when it is buried and cemented. Deep burial or the collision of tectonic plates can heat and squeeze rocks of any kind until their minerals recrystallize, producing metamorphic rock. Push the heat far enough and rock melts, starting the loop again.
There is no fixed order. An igneous rock can be metamorphosed without ever becoming sediment, and a metamorphic rock can be lifted and eroded straight away. Plate tectonics powers the whole system, which runs over millions to billions of years.
Gems in igneous rocks
Igneous settings produce some of the most important gems. Diamonds crystallize deep in the mantle and reach the surface in kimberlite and lamproite eruptions. Basalt lavas carry sapphire and zircon crystals up from depth and bring peridot from the mantle. Granite pegmatites, the coarse final stage of a granite magma, hold large crystals of tourmaline, beryl, topaz and spodumene.
The igneous rocks and gemstones page covers each of these settings.
Gems in metamorphic rocks
Metamorphism grows new minerals as rock adjusts to heat and pressure. Marble formed from limestone hosts the finest ruby and spinel, including the deposits of Myanmar's Mogok tract. Schists host many emeralds, garnets and kyanite. Jadeite forms at high pressure and fairly low temperature in subduction zones, and lapis lazuli grows where marble reacts with nearby rocks.
Read more on the metamorphic rocks and gemstones page.
Gems in sedimentary settings
Sedimentary processes form some gems and concentrate others. Opal in Australia formed from silica-rich water seeping through weathered sedimentary rock. Turquoise and malachite grow near the surface where water alters copper deposits. Amber, fossil tree resin, is preserved in sedimentary layers. Rivers sort hard, heavy gems from eroded rock into gravels, which is how most Sri Lankan sapphire and much of the world's early diamond supply was found.
The sedimentary rocks and gemstones page covers these deposits.
Hydrothermal veins and pegmatites
Two settings cut across the three families and deserve their own mention.
Pegmatites form from the last, water-rich fluid left after a granite magma crystallizes. That fluid concentrates rare elements such as beryllium, lithium and boron, which the main rock could not absorb. Crystals can grow very large in open pockets, giving gem tourmaline, aquamarine, morganite, topaz and kunzite. Many of the gem pegmatites of Brazil's Minas Gerais, Madagascar, Afghanistan and southern California formed this way.
Hydrothermal veins form when hot, mineral-rich water moves through fractures and drops minerals as it cools or reacts with the rock around it. Colombian emeralds are the best-known gem example. They grew in calcite and pyrite veins cutting through black shale, a sedimentary rock, where hot brines supplied the beryllium, chromium and vanadium. Amethyst geodes in Brazilian and Uruguayan basalts also formed from fluids moving through gas cavities in the cooled lava.
Host-rock and alluvial deposits
Geologists separate deposits into two kinds. Deposits in the host rock are found where the gem first grew, such as a kimberlite pipe or a pegmatite pocket. Alluvial or placer deposits form after erosion frees gems from their host rock and water carries them away. Because gems such as sapphire, diamond and spinel are hard and dense, they survive transport and settle in river bends and gravel beds while lighter, softer minerals wash on. Many of the world's gem fields, including Sri Lanka, are alluvial.
Frequently asked questions
Which type of rock has the most gemstones?
All three families host gems. Igneous pegmatites and metamorphic marbles and schists produce many of the major gem species, while sedimentary gravels concentrate gems that formed elsewhere.
How long does the rock cycle take?
It runs on geological timescales, from millions to hundreds of millions of years per loop. Some diamonds are more than a billion years old.
Can a gem change from one rock type to another?
The gem itself usually survives as a durable grain, but its host rock can change. Garnets in a schist can weather out and collect in river sand.