Metamorphic rocks form when existing rocks are changed by heat, pressure and chemically active fluids without fully melting. As the conditions change, old minerals break down and new ones grow, sometimes as gem crystals. Marble hosts the finest ruby and spinel, schists host many emeralds and garnets, and high-pressure rocks produce jadeite and kyanite.
How metamorphism works
Rocks are stable only within a certain range of temperature and pressure. When a rock is buried deeper, squeezed between colliding plates or baked by nearby magma, its minerals react to form new ones that are stable under the new conditions. The rock stays solid the whole time. Atoms move through grain boundaries and thin films of fluid, and crystals grow slowly, often over millions of years.
Geologists describe several main types:
- Regional metamorphism affects large areas during mountain building, when plates collide and rocks are buried and folded. Most schists and gneisses form this way.
- Contact metamorphism happens next to a body of hot magma. The rock nearest the intrusion is baked the most, forming zones of different minerals.
- Metasomatism is metamorphism in which fluids add or remove chemical elements. Many gem deposits form where fluids carry elements between two very different rocks, such as granite and marble.
- High-pressure, low-temperature metamorphism happens in subduction zones, where cold oceanic crust sinks fast enough to stay relatively cool under great pressure.
The starting rock matters as much as the conditions. Limestone becomes marble, shale becomes slate, then schist, then gneiss as temperature rises, and sandstone becomes quartzite.
Index minerals and metamorphic grade
Certain minerals grow only within particular ranges of temperature and pressure, so geologists use them as markers of metamorphic grade. In shale-derived rocks, rising temperature produces a classic sequence: chlorite, then biotite, then garnet, then staurolite, then kyanite, then sillimanite. Kyanite, andalusite and sillimanite all have the same chemistry (aluminum silicate), and which one forms depends on pressure and temperature. Kyanite is the high-pressure form.
Marble-hosted ruby and spinel
Many of the world's finest rubies grow in marble, which forms from limestone during regional metamorphism. Ruby needs aluminum and chromium and very little silica and iron. Impure limestones can supply the aluminum and chromium, while the marble's low iron content allows a bright red color and strong red fluorescence. Iron tends to dampen that fluorescence, which is one reason marble-hosted rubies often glow more than rubies from iron-rich basalt settings.
The Mogok Stone Tract in Myanmar is the classic marble-hosted ruby field, described on the Mogok ruby page. Similar deposits occur in Vietnam's Luc Yen district, Afghanistan's Jegdalek area and the Pamir mountains of Tajikistan. These same marbles also produce red, pink and purple spinel, which is why spinel and ruby were confused for centuries.
Schist-hosted emerald
Emerald needs beryllium, which is concentrated in granites and pegmatites, and chromium or vanadium, which come from dark mafic and ultramafic rocks. Most of the world's emerald deposits form where granite-related fluids rich in beryllium meet chromium-bearing rocks. The contact zone becomes a dark, mica-rich schist (often called blackwall or phlogopite schist) with emerald crystals.
This model explains emerald deposits in Zambia's Kafubu area, Brazil, Russia's Ural Mountains, Zimbabwe's Sandawana and Pakistan's Swat Valley. Colombian emeralds are the main exception. They formed from hot brines in veins cutting black shale, with no granite involved.
Garnet, tanzanite and other schist gems
Garnet is one of the most common metamorphic minerals. Almandine garnet grows as rounded dodecahedra in mica schist and gneiss, and the almandine garnet page covers its uses. Grossular garnet forms in metamorphosed limestones and calc-silicate rocks. Green tsavorite, a grossular, grows in graphite-bearing gneisses in Kenya and Tanzania.
Tanzanite also comes from metamorphic rock. It occurs only in the Merelani Hills of northern Tanzania, where zoisite grew in folded graphitic gneiss and schist during regional metamorphism. Iolite (gem cordierite), sillimanite and some sapphire also come from high-grade metamorphic rocks, and much of Sri Lanka's gem wealth eroded from them.
Jade and subduction zones
Jadeite forms at high pressure and relatively low temperature, conditions typical of subduction zones. Fluids moving through serpentinite in these zones deposit jadeite as veins and blocks. Myanmar's Hpakant area is the main source of fine jadeite, and other deposits occur in Guatemala, Japan and Russia.
Nephrite, the other jade, forms by metasomatism, usually where serpentinite meets other rock or where dolomite marble meets granite. Its tightly matted amphibole fibers make it extremely tough. Nephrite sources include China, New Zealand, British Columbia and Siberia.
Lapis lazuli
Lapis lazuli is a metamorphic rock in its own right. It forms by contact metamorphism and metasomatism of marble, where fluids add sodium, sulfur and other elements that build the blue mineral lazurite. Calcite remains as white streaks, and pyrite adds gold flecks. The Sar-e-Sang deposits in Afghanistan's Badakhshan province have been worked for thousands of years, as covered on the gems of Afghanistan page.
Summary of metamorphic gems
| Gem | Host rock | Type of metamorphism | Example sources |
|---|---|---|---|
| Ruby and spinel | Marble | Regional | Myanmar (Mogok), Vietnam, Afghanistan, Tajikistan |
| Emerald | Mica schist at granite contacts | Metasomatic | Zambia, Brazil, Russia, Pakistan |
| Almandine garnet | Mica schist, gneiss | Regional | India, Sri Lanka, Brazil, many others |
| Tsavorite and tanzanite | Graphitic gneiss | Regional | Kenya, Tanzania |
| Jadeite | Serpentinite-related veins | High-pressure, low-temperature | Myanmar, Guatemala |
| Nephrite | Serpentinite and marble contacts | Metasomatic | China, New Zealand, Canada |
| Lapis lazuli | Altered marble | Contact and metasomatic | Afghanistan, Russia, Chile |
| Kyanite | Schist, gneiss | Regional, higher pressure | Nepal, Brazil, India |
From metamorphic rock to river gravel
Metamorphic gems that are hard and dense, such as ruby, sapphire, spinel and garnet, survive when their host rocks weather away. Rivers carry them into gravels, joining the sedimentary stage of the cycle. Much of the ruby mined at Mogok and most Sri Lankan gems come from these gravels, even though they formed in metamorphic rock.
Frequently asked questions
Why do the best rubies come from marble?
Marble is low in iron, so marble-hosted rubies tend to show bright red color and strong red fluorescence. Iron in other host rocks tends to dull both.
Is emerald a metamorphic gem?
Most emerald deposits are metamorphic, forming where beryllium-rich fluids reached chromium-bearing rocks and created mica schists. Colombian emerald is the exception and formed in hydrothermal veins in shale.
What is metasomatism?
Metasomatism is metamorphism in which fluids add or remove elements from a rock. It forms many gem deposits, including nephrite, lapis lazuli and most emeralds.
Where does jadeite form?
Jadeite forms at high pressure and fairly low temperature in subduction zones, usually in rocks linked to serpentinite.