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Peridot is the gem-quality variety of the mineral olivine. It is an idiochromatic mineral, meaning its color comes from an integral part of its chemical composition, specifically iron (Fe2+). The characteristic green color ranges from yellowish-green to pure green to brownish-green, with the most prized color being a vibrant, intense green without yellowish or brownish tints. It is one of the few gemstones that occurs in only one color. Peridot is known for its high birefringence, which can cause a 'sleepy' or 'velvety' appearance in larger stones, and its strong double refraction, often visible as a doubling of facet edges when viewed through the stone.
How to Identify
- Color
- Ranges from yellowish-green to pure green to brownish-green. The most desirable color is a vivid, intense green.
- Luster
- Vitreous (glassy) to oily.
- Texture
- Smooth to the touch when polished; rough in its natural crystal form.
- Crystal Form
- Orthorhombic system, typically forming stubby prismatic crystals, often with striations. Can also be found as granular aggregates.
- Cleavage
- Poor to indistinct in two directions (parallel to {010} and {110}). Fracture is conchoidal.
- Geological Environment
- Found in mafic and ultramafic igneous rocks (e.g., basalts, peridotites, dunites), especially as xenoliths in volcanic rocks. Also occurs in some metamorphic rocks and pallasite meteorites.
Key Facts
- Hardness: 6.5-7 on Mohs scale
- Specific Gravity: 3.27-3.48
- Crystal System: Orthorhombic
- Color: Green (yellowish-green, pure 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 pure green to brownish-green
- Luster: Vitreous to oily
- Streak: White
Physical Characteristics
- Crystal Habit: Prismatic, tabular, granular, massive
- Cleavage Type: Poor to indistinct, {010} and {110}
- Fracture Type: Conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous to oily
Formation
Peridot, as a gem-quality variety of olivine, forms under high-temperature and high-pressure conditions. The primary geological environments for its formation include: 1. Crystallization from mafic and ultramafic magmas (e.g., basalts, peridotites, dunites) deep within the Earth's mantle, brought to the surface by volcanic activity. 2. Metamorphism of impure dolomitic limestones or other magnesium-rich rocks. 3. Extraterrestrial origin, found in pallasite meteorites, where it crystallized from a molten iron-nickel matrix.
Usage
Primarily used as a gemstone in jewelry due to its attractive green color and vitreous luster. Historically, it was used for ornamental purposes and believed to have protective powers. Industrial olivine (non-gem quality) is used in refractories, as a foundry sand, and as an abrasive.
Age Distribution
Peridot forms in various geological settings, from the Earth's mantle to extraterrestrial sources, thus its age can range from billions of years (mantle xenoliths) to millions of years (volcanic rocks) or even younger (meteorites).
Where to Find
San Carlos Apache Reservation, Arizona, USA
One of the most significant sources of gem-quality peridot, particularly large stones. Found in basaltic lava flows.
Zabargad Island (St. John's Island), Egypt
Historically important source, known since ancient times. Peridot here is found in peridotite xenoliths within basaltic rocks.
Myanmar (Burma)
Produces fine quality peridot, often found in association with serpentine and other ultramafic rocks.
Pakistan (Kaghan Valley)
Known for producing high-quality, vibrant green peridot, often found in association with serpentinite.
China
Significant producer, with deposits found in basaltic rocks.
Vietnam
Emerging source of good quality peridot.
Norway
Found in some ultramafic intrusions.
Australia (Queensland)
Occurs in basaltic rocks.
Finding Tips
Geological Context
Focus on areas with recent or ancient volcanic activity, particularly basaltic lava flows or ultramafic intrusions. Look for xenoliths (rock fragments) within these igneous rocks.
Associated Minerals
Peridot is often found with other mantle minerals like pyroxenes, chromite, and magnetite in its primary geological settings.
Color and Luster
Look for transparent to translucent green crystals with a vitreous luster. The distinctive green color is a key identifier.
Double Refraction
In larger, transparent specimens, the strong double refraction of peridot can be observed by looking through the stone at a light source or printed text; facet edges or text will appear doubled.
Hardness Test (Caution)
While not recommended for valuable specimens, peridot's hardness of 6.5-7 on the Mohs scale means it can scratch glass but can be scratched by quartz. Use this only on non-gem quality or small fragments.
Similar Rocks
Green Garnet (Grossular or Demantoid)
Grossular Garnet, Andradite Garnet
Also known as: Tsavorite (grossular), Uralian Emerald (demantoid)
Green Tourmaline
Elbaite (Tourmaline group)
Also known as: Verdelite
Emerald
Beryl
Also known as: Beryl (green variety)
Diopside
Diopside
Also known as: Chrome Diopside
Scientific Classification
- Mineral Class
- Nesosilicate
- Group
- Olivine Group
- Crystal System
- Orthorhombic
- Chemical Formula
- (Mg,Fe)2SiO4
- Composition
- Magnesium iron silicate. Peridot is the gem-quality variety of forsterite (Mg2SiO4) and fayalite (Fe2SiO4) solid solution series, with the iron content typically ranging from 5% to 20% for gem quality material. The green color is due to the presence of ferrous iron (Fe2+).
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