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Peridot is the gem-quality variety of the mineral olivine, a nesosilicate. It is characterized by its distinctive yellowish-green to greenish-yellow color, which is caused by the presence of iron within its crystal structure. Peridot is one of the few gemstones that occurs in only one color, though the intensity and tint of the green can vary. It crystallizes in the orthorhombic system and typically forms stubby prismatic crystals, granular aggregates, or massive forms. It is a relatively hard mineral but can be brittle. Peridot is a significant component of the Earth's upper mantle.
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
- Often granular in massive forms; individual crystals are typically stubby prismatic.
- Crystal Form
- Orthorhombic system; typically short, stout prismatic crystals, often with striations. Also found as rounded grains or massive aggregates.
- Cleavage
- Poor to indistinct in two directions (parallel to {010} and {110}), often exhibiting conchoidal fracture more readily than cleavage.
- Geological Environment
- Mafic and ultramafic igneous rocks (basalt, gabbro, peridotite, dunite), mantle xenoliths, certain metamorphic rocks (dolomitic marbles), and pallasite meteorites.
Key Facts
- Hardness: 6.5-7 on Mohs scale
- Specific Gravity: 3.27-3.37 (increases with Fe content)
- Crystal System: Orthorhombic
- Color: Yellowish-green, olive green, brownish-green
- Luster: Vitreous to oily
- Transparency: Transparent to translucent
- Fracture: Conchoidal
- Cleavage: Poor to indistinct in two directions ({010} and {110})
- Composition: Magnesium iron silicate (Mg,Fe)2SiO4
Quick Check
- Color: Yellowish-green to green
- Luster: Vitreous to oily
- Streak: White
Physical Characteristics
- Crystal Habit: Stubby prismatic crystals, granular, massive
- Cleavage Type: Poor to indistinct, {010} and {110}
- Fracture Type: Conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous to oily
Formation
Peridot is the gem-quality variety of the mineral olivine. Olivine forms at high temperatures and pressures, typically crystallizing from magnesium-rich, silica-poor magmas deep within the Earth's mantle. It is a primary mineral in mafic and ultramafic igneous rocks such as basalt, gabbro, peridotite, and dunite. Peridot can also be found in certain metamorphic rocks (e.g., dolomitic marbles) and as xenocrysts in volcanic eruptions, brought to the surface from the mantle. Extraterrestrial peridot has been found in pallasite meteorites.
Usage
Gemstone (birthstone for August), industrial abrasive (olivine sand), refractory material (due to high melting point), metallurgical flux, and as a source of magnesium. Scientific research uses include mantle studies and CO2 sequestration experiments.
Age Distribution
Olivine is a primary mineral in many mafic and ultramafic igneous rocks, which can range from Archean to Cenozoic in age. Gem-quality peridot is often found in Cenozoic basalts or mantle xenoliths.
Where to Find
San Carlos Apache Reservation, Arizona, USA
One of the most significant sources of gem-quality peridot, found in basaltic lava flows and volcanic bombs.
Zabargad Island (St. John's Island), Red Sea, Egypt
Historically important source, known for producing large, fine-quality peridot crystals from peridotite xenoliths.
Myanmar (Burma)
Produces high-quality peridot, often found in association with serpentine and other ultramafic rocks.
Pakistan
Known for producing fine, vibrant green peridot, particularly from the Supat Valley.
China
Significant producer of peridot, often found in basaltic rocks.
Vietnam
Emerging source of peridot, found in basaltic terrains.
Finding Tips
Look in Volcanic Areas
Peridot is commonly found in basaltic lava flows and volcanic bombs, especially in areas with recent volcanic activity. Look for dark, vesicular volcanic rocks.
Examine Ultramafic Rocks
In areas with exposed mantle rocks (peridotites, dunites), peridot can be a primary constituent. These rocks are often dark green to black.
Check Alluvial Deposits
Due to its relative hardness and resistance to weathering, peridot can be found in placer deposits (riverbeds, gravels) downstream from its primary sources.
Distinguish from Similar Minerals
Pay attention to the distinct yellowish-green color, vitreous luster, and lack of strong cleavage. Green garnets (e.g., grossular) are typically harder and have different crystal habits. Diopside often has better cleavage.
Use a Magnifying Glass
Examine the crystal habit and internal features. Peridot often shows 'lily pad' inclusions (stress fractures) or chromite crystals.
Similar Rocks
Diopside
CaMgSi2O6
Also known as: Chrome Diopside (gem variety)
Grossular Garnet
Ca3Al2(SiO4)3
Also known as: Tsavorite (green variety)
Green Tourmaline
Complex borosilicate
Also known as: Verdelite
Scientific Classification
- Mineral Class
- Silicates
- Group
- Nesosilicates (Olivine Group)
- Crystal System
- Orthorhombic
- Chemical Formula
- (Mg,Fe)2SiO4
- Composition
- Magnesium iron silicate. The end-members are forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Peridot is an intermediate member with a higher Mg content than Fe.
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