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Peridot is the gem-quality variety of the mineral olivine. It is an idiochromatic mineral, meaning its color comes from its essential chemical composition rather than from trace impurities. The characteristic yellowish-green to greenish-yellow color is due to the presence of iron (Fe2+) within its crystal structure. It is one of the few gemstones that occurs in only one color, though the intensity and tint of the green can vary. Peridot is known for its relatively high birefringence, which can cause a slightly fuzzy or 'sleepy' appearance to facet edges when viewed under magnification. It is a nesosilicate, meaning its crystal structure consists of isolated SiO4 tetrahedra.
How to Identify
- Color
- Typically yellowish-green to pure green, sometimes brownish-green. The intensity of green depends on the iron content.
- Luster
- Vitreous (glassy) to oily.
- Texture
- Smooth to the touch when polished; rougher in its natural crystal form.
- Crystal Form
- Orthorhombic system, often found as stubby prismatic crystals, granular aggregates, or rounded grains in volcanic rocks. Well-formed crystals are relatively rare.
- Cleavage
- Poor to distinct in two directions at 90 degrees, but typically exhibits conchoidal fracture rather than clean cleavage.
- Geological Environment
- Found in mafic and ultramafic igneous rocks (basalts, peridotites, dunites), kimberlites, and as xenoliths in volcanic eruptions. Also found in pallasite meteorites.
Key Facts
- Hardness: 6.5-7 on the Mohs scale
- Specific Gravity: 3.27-3.48 (typically 3.34)
- Crystal System: Orthorhombic
- Color: Yellowish-green, olive green, brownish-green
- Luster: Vitreous to oily
- Transparency: Transparent to translucent
- Fracture: Conchoidal
- Cleavage: Poor to distinct in two directions (010) and (110), but rarely observed due to conchoidal fracture
- Composition: Magnesium iron silicate, (Mg,Fe)2SiO4
Quick Check
- Color: Yellowish-green to pure green
- Luster: Vitreous (glassy) to oily
- Streak: White
Physical Characteristics
- Crystal Habit: Prismatic, tabular, granular, massive. Often found as rounded grains in volcanic rocks.
- Cleavage Type: Poor to distinct on {010} and {110}, but fracture is more common.
- Fracture Type: Conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous to oily
Formation
Peridot, the gem variety of olivine, forms in high-temperature, low-silica environments. It crystallizes from magma deep within the Earth's mantle (typically 20-90 km deep) and is brought to the surface through volcanic activity, often found in basaltic lavas, peridotites, and kimberlites. It can also be found in certain types of meteorites (pallasites), indicating extraterrestrial formation.
Usage
Primarily used as a gemstone in jewelry due to its attractive green color and vitreous luster. Historically, it was used for talismans and decorative objects. Industrial olivine (non-gem quality) is used in refractories, as a foundry sand, and as an abrasive.
Age Distribution
Formed throughout Earth's history, often associated with mantle-derived rocks, some extraterrestrial occurrences (pallasite meteorites) are billions of years old.
Where to Find
San Carlos Apache Reservation, Arizona, USA
One of the most significant sources of peridot, particularly large quantities of smaller stones. Found in basaltic lava flows.
Zabargad Island (St. John's Island), Red Sea, Egypt
Historically important source, known for producing large, fine-quality peridot. Associated with mantle peridotites.
Kashmir, Pakistan
Produces high-quality, vibrant green peridot, often found in association with serpentinite.
Myanmar (Burma)
Known for producing some of the finest, large peridot crystals, often found in alluvial deposits derived from peridotite intrusions.
China
Significant producer of peridot, particularly from Hebei and Jilin provinces, often found in basaltic rocks.
Vietnam
Emerging source of good quality peridot.
Norway
Historically known for peridot, particularly from the island of Almklovdalen.
Finding Tips
Look in Volcanic Areas
Peridot is commonly found in basaltic lava flows and volcanic bombs. Search in areas with recent or ancient volcanic activity, especially where erosion has exposed the underlying rock.
Check Ultramafic Rocks
Explore outcrops of peridotite, dunite, and serpentinite, which are primary host rocks for olivine. Peridot can occur as disseminated grains or larger crystals within these rocks.
Examine Alluvial Deposits
Due to its relative hardness and resistance to weathering, peridot can be found in riverbeds and gravel deposits downstream from its primary source. Look for rounded, greenish pebbles.
Identify by Color and Luster
Peridot's distinctive yellowish-green to pure green color and vitreous luster are key identifiers. Look for transparent to translucent grains or crystals.
Consider Meteorite Hunting
While rare, peridot (olivine) is a major component of pallasite meteorites. If you are involved in meteorite hunting, keep an eye out for metallic meteorites with embedded green crystals.
Similar Rocks
Green Garnet
Andradite, Grossular (varieties of Garnet)
Also known as: Demantoid, Tsavorite
Green Tourmaline
Elbaite (variety of Tourmaline)
Also known as: Verdelite
Emerald
Beryl
Also known as: Beryl (green variety)
Diopside
Diopside
Also known as: Chrome Diopside (gem variety)
Scientific Classification
- Mineral Class
- Nesosilicate
- Group
- Olivine Group
- Crystal System
- Orthorhombic
- Chemical Formula
- (Mg,Fe)2SiO4
- Composition
- Magnesium iron silicate. Peridot is the gem variety of forsterite (Mg2SiO4) and fayalite (Fe2SiO4) solid solution series, with peridot being rich in magnesium (forsterite end-member).
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