igneous
Peridot in Basalt/Xenolith
Olivine (Mg, Fe)2SiO4 in Vesicular Basalt
Hardness: 6.5-7 (Olivine), 5-6 (Basalt); Color: Olive green to lime green crystals within a dark grey or black matrix; Luster: Vitreous (glassy) for the crystals, dull/earthy for the matrix; Structure: Orthorhombic crystals appearing as granular masses; Specific Gravity: 3.2-4.4.
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Olivine), 5-6 (Basalt); Color: Olive green to lime green crystals within a dark grey or black matrix; Luster: Vitreous (glassy) for the crystals, dull/earthy for the matrix; Structure: Orthorhombic crystals appearing as granular masses; Specific Gravity: 3.2-4.4.
Formation & geological history
Formed deep within the Earth's upper mantle and brought to the surface via volcanic eruptions. The green olivine crystals (xenoliths) were trapped in the basaltic lava as it cooled rapidly on or near the surface. Most are from the Cenozoic era or younger.
Uses & applications
Larger, clear crystals are faceted into Peridot jewelry (August birthstone). Smaller specimens are common in geological collecting and education. Basalt itself is used extensively in construction and road aggregate.
Geological facts
Olivine is one of the first minerals to crystallize from magma. Large pockets of these crystals are found in the San Carlos Apache Reservation in Arizona, which provides much of the world's gem-quality peridot. Examples have also been found in meteorites known as pallasites.
Field identification & locations
Identify by looking for glassy, olive-green granular aggregates embedded in a fine-grained, dark, often holy (vesicular) volcanic rock. Common in Hawaii, Arizona, and volcanic regions of New Mexico and Canary Islands. Collectors should look for vibrant green color and high clarity in the crystals.
