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Peridot is the gem-quality variety of the mineral olivine. Olivine is a magnesium iron silicate mineral with the chemical formula (Mg,Fe)2SiO4. It is one of the most abundant minerals in the Earth's upper mantle. Peridot is characterized by its distinctive yellowish-green to pure green color, which is caused by the presence of iron within its crystal structure. It is idiochromatic, meaning its color comes from an essential component of its chemical composition rather than trace impurities. Olivine is a nesosilicate, meaning its crystal structure consists of isolated SiO4 tetrahedra linked by divalent cations (Mg, Fe).
How to Identify
- Color
- Typically yellowish-green to pure green, sometimes olive-green or brownish-green. The intensity of green depends on the iron content.
- Luster
- Vitreous (glassy) to oily.
- Texture
- Often granular or massive. Individual crystals are typically euhedral to subhedral, often with a somewhat rounded appearance in volcanic rocks.
- Crystal Form
- Orthorhombic system. Crystals are typically short prismatic, often tabular or equant. In granular aggregates, individual grains are anhedral to subhedral. Gem-quality crystals can be well-formed.
- Cleavage
- Poor to indistinct in two directions (parallel to {010} and {110}), often appearing as conchoidal fracture rather than true cleavage. This poor cleavage is a key distinguishing feature.
- Geological Environment
- Found in mafic and ultramafic igneous rocks (e.g., basalt, gabbro, peridotite, dunite), metamorphic rocks derived from these, and as xenoliths in volcanic rocks. Also found in some meteorites.
Key Facts
- Hardness: 6.5-7 on the Mohs scale
- Specific Gravity: 3.27-3.37 (higher with increasing iron content)
- Crystal System: Orthorhombic
- Color: Yellowish-green, olive-green, pure green, brownish-green
- Luster: Vitreous to oily
- Transparency: Transparent to translucent
- Fracture: Conchoidal to uneven
- Cleavage: Poor to indistinct in two directions ({010} and {110})
- Composition: Magnesium iron silicate, (Mg,Fe)2SiO4
Quick Check
- Color: Yellowish-green to pure green
- Luster: Vitreous to oily
- Streak: White
Physical Characteristics
- Crystal Habit: Short prismatic, tabular, equant, granular, massive
- Cleavage Type: Poor to indistinct, often appearing as fracture
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous (glassy) to oily
Formation
Olivine forms at high temperatures and pressures, typically crystallizing from mafic and ultramafic magmas deep within the Earth's mantle or lower crust. It is a major component of peridotite and basalt. Gem-quality peridot often forms in basaltic lavas or within mantle xenoliths brought to the surface by volcanic activity. It can also be found in some meteorites (pallasites).
Usage
Gem-quality peridot is primarily used as a gemstone in jewelry. Industrial-grade olivine is used as a refractory material, a slag conditioner in metallurgy, a sandblasting abrasive, and as a source of magnesium. It is also explored for carbon sequestration due to its reactivity with CO2.
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 basaltic lavas 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 lavas.
Zabargad Island (St. John's Island), Red Sea, Egypt
Historically important source, known for fine-quality peridot, found in peridotite xenoliths.
Myanmar (Burma)
Produces high-quality, large peridot crystals, often found in association with serpentine.
Pakistan (Kaghan Valley)
Known for producing vibrant green peridot, often found in peridotite xenoliths within volcanic rocks.
China
Significant producer of peridot, particularly in the Changbai Mountains region.
Norway (Åheim)
Major industrial source of olivine, primarily for refractory and metallurgical uses, found in dunite.
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 rocks.
Examine Ultramafic Rocks
In areas with exposed peridotite or dunite (ultramafic igneous rocks), olivine can be a major constituent. These rocks are typically dark green to black.
Check Stream Beds and Alluvial Deposits
Due to its relative hardness and density, peridot can be found as alluvial pebbles in stream beds downstream from its primary source.
Identify Associated Minerals
Peridot is often found with other mafic minerals like pyroxene, plagioclase feldspar, and magnetite.
Beware of Alteration
Olivine readily alters to serpentine, iddingsite, or chlorite, especially in the presence of water. Fresh, unaltered olivine is more desirable for collection.
Similar Rocks
Serpentine
Serpentine Group Minerals
Also known as: Antigorite, Lizardite, Chrysotile
Epidote
Epidote
Also known as: Pistacite
Diopside
Diopside
Also known as: Chrome Diopside
Scientific Classification
- Mineral Class
- Silicates
- Group
- Nesosilicates (Orthosilicates)
- Crystal System
- Orthorhombic
- Chemical Formula
- (Mg,Fe)2SiO4
- Composition
- Magnesium iron silicate, forming a solid solution series between forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Peridot is typically an intermediate member, rich in magnesium (forsterite-rich).
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