Volcanic Gemstones: Gems Born From Lava

## Fire and Crystal Volcanic activity creates and delivers some of the world's most important gemstones. Some crystallize directly from cooling magma. Others form deep in the mantle and hitchhike to the surface in volcanic eruptions. A few are the volcanic glass itself — magma that cooled too quickly for crystals to form. ## Peridot: The Mantle Gemstone Peridot is gem-quality olivine — a magnesium iron silicate that is one of the most abundant minerals in Earth's upper mantle. Unlike most gemstones that form in the crust, peridot originates in the mantle at depths of 20-55 kilometers and reaches the surface through volcanic eruptions. Basaltic volcanism carries xenoliths (chunks of mantle rock) containing peridot crystals. The peridot is extracted from these xenoliths after erosion exposes them. Arizona's San Carlos Reservation produces peridot from basaltic lava flows where mantle xenoliths weather out of the rock. The finest peridots come from Pakistan's Kohistan district and Myanmar's Mogok region, where larger crystals with vivid green color are found in serpentinized dunite and peridotite. | Source | Character | Notable Quality | |--------|-----------|----------------| | Pakistan (Kohistan) | Deep olive green, large crystals | Finest available | | Myanmar (Mogok) | Vivid green | Premium | | Arizona (San Carlos) | Yellowish green, small | Commercial | | China (Changbai) | Variable | Growing production | | Egypt (Zabargad Island) | Historical source | Exhausted | Peridot's characteristic color comes from iron in its crystal structure. Unlike most green gems colored by chromium or vanadium, peridot's green is idiochromatic — inherent to the mineral itself. ## Obsidian: Volcanic Glass Obsidian forms when silica-rich lava cools too rapidly for crystals to develop, producing a natural glass. While not a crystal, obsidian has been used as a gemstone and tool material for thousands of years. Varieties include: - **Black obsidian**: The most common form - **Snowflake obsidian**: Black with white cristobalite inclusions - **Mahogany obsidian**: Black and brown banded - **Rainbow obsidian**: Displays iridescent bands caused by nanoparticle inclusions - **Fire obsidian**: Rare, displays thin-film interference colors similar to opal Obsidian is not durable for fine jewelry (5-5.5 Mohs) but is widely used in decorative carving and fashion jewelry. ## Zircon: Not Cubic Zirconia Zircon (zirconium silicate) is a genuine gemstone that crystallizes from silica-rich magmas, particularly granites and syenites. It is not to be confused with cubic zirconia (CZ), which is a lab-created diamond simulant. Volcanic zircon is among the oldest minerals on Earth. Zircon crystals from Australia's Jack Hills have been dated to 4.4 billion years — nearly as old as Earth itself. These crystals contain evidence that liquid water existed on early Earth. Gem zircon occurs in multiple colors: blue (heat-treated), golden, red-brown, green, and colorless. Blue zircon from Cambodia and Myanmar is the most popular variety, displaying exceptional brilliance (RI 1.93-1.98) and fire (dispersion 0.039) that approaches diamond. ## Topaz and Beryl From Pegmatites While not directly volcanic, pegmatites form from the late-stage crystallization of granitic magma. As the magma cools, volatile-rich fluids concentrate rare elements and produce exceptionally large crystals. Pegmatite environments produce: - **Topaz**: Crystallizes from fluorine-rich fluids in the final stages of pegmatite cooling. Imperial topaz from Ouro Preto, Brazil, forms in kaolinized rhyolite (weathered volcanic rock) - **Aquamarine and Emerald**: Form in beryl-bearing pegmatites where beryllium concentrates - **Tourmaline**: The complex boron chemistry of tourmaline thrives in pegmatite environments - **Kunzite (spodumene)**: Found in lithium-rich pegmatites ## Labradorite: The Iridescent Feldspar Labradorite crystallizes from basaltic and gabbroic magma. Its famous labradorescence — flashes of blue, green, gold, and orange — comes from light reflecting off alternating layers of different feldspar compositions that form during slow cooling. The phenomenon is named after Labrador, Canada, where it was first identified. Spectrolite, a variety from Finland, shows a full spectrum of colors and is valued as a collector gem and decorative material. ## Why Volcanic Gems Are Different Volcanic gemstones share characteristics that distinguish them from metamorphic and hydrothermal gems: - They often contain gas bubbles and flow textures (obsidian) - They may include mantle minerals not found in crustal rocks (peridot) - Crystal sizes tend to be smaller (rapid cooling limits growth), except in pegmatites - They are geographically tied to volcanic terrains and tectonic boundaries ## Modern Volcanic Gems: Tanzania and East Africa The East African Rift System is a modern zone of volcanic activity that has produced important gemstone deposits. Tanzanite formed in graphitic gneiss metamorphosed by rift-related geological activity near Mount Kilimanjaro. Tsavorite garnet occurs in rift-associated metamorphic rocks in Kenya and Tanzania. The rift volcanism continues today, and new gem deposits may emerge as geological exploration continues. ## Identifying Volcanic Origin in Gems Gemologists can sometimes identify volcanic origin through specific inclusion types: - Gas-liquid two-phase inclusions in volcanic environments - Mantle minerals (olivine, pyroxene) in peridot - Flow textures and spherulites in obsidian - High-temperature mineral inclusions indicating magmatic origin