Overview of Peridot

Peridot is the gem-quality variety of the mineral olivine, with the idealized chemical formula (Mg,Fe)₂SiO₄. It is one of the few gemstones that occurs in only one color family—green—though its exact shade ranges from yellowish-green to deep olive depending on iron content. Peridot has been valued as a gemstone for thousands of years and is one of the oldest known gems used in jewelry.

Unlike many gemstones that owe their color to trace impurities, peridot’s green color is intrinsic to its chemical composition, caused primarily by iron within the crystal structure. Searches such as “what is peridot,” “peridot vs olivine,” and “where is peridot found” reflect its importance in both gemology and geology.

Peridot forms deep within the Earth’s mantle and is brought to the surface through volcanic processes, making it not only a gemstone but also a window into Earth’s interior.

Chemical Composition and Classification

The chemical formula of peridot (olivine) is:

(Mg,Fe)₂SiO₄

It belongs to:

• Mineral Class: Silicates
• Subclass: Nesosilicates (independent tetrahedra)
• Group: Olivine group

Peridot is the magnesium-rich variety of olivine. The end-member compositions are:

• Forsterite (Mg₂SiO₄) – magnesium-dominant
• Fayalite (Fe₂SiO₄) – iron-dominant

Gem-quality peridot typically has a higher magnesium content (forsteritic composition) with moderate iron, which gives it its green color.

Peridot is non-radioactive and chemically stable under surface conditions, though it can alter to serpentine or iddingsite over geological time.

Crystal Structure and Physical Properties

Peridot crystallizes in the orthorhombic crystal system.

Physical properties of peridot include:

• Crystal system: Orthorhombic
• Habit: Short prismatic crystals, granular masses
• Color: Yellow-green, olive-green, brownish-green
• Streak: Colorless
• Luster: Vitreous
• Hardness: 6.5–7 on the Mohs scale
• Cleavage: Poor
• Fracture: Conchoidal to uneven
• Specific gravity: Approximately 3.2–3.4

Peridot often displays:

• Strong double refraction (visible doubling of facet edges in cut stones)
• Slight pleochroism (variation in green tones)

Its relatively high refractive index gives it good brilliance when faceted.

Formation and Geological Environment

Peridot forms in mantle-derived rocks and volcanic environments.

Primary formation settings include:

• Upper mantle peridotite
• Basaltic lava flows
• Kimberlite pipes
• Ultramafic intrusions

Peridot crystals are often brought to the surface by volcanic eruptions. They may occur as:

• Xenocrysts (mantle fragments carried upward by magma)
• Phenocrysts within basalt

Peridot can also form in some meteorites (pallasites), making it one of the few gemstones known to occur extraterrestrially.

Locations and Notable Deposits

Major peridot sources include:

• Arizona, USA (San Carlos Reservation) – Major commercial source
• Pakistan (Kashmir region) – Fine, deep green stones
• Myanmar (Burma) – Historic deposits
• China: Significant modern producer
• Egypt (Zabargad Island/St. John’s Island) – Ancient source

The Egyptian deposits were known in antiquity and supplied peridot to early Mediterranean civilizations.

Associated Minerals

Peridot occurs with:

• Pyroxene
• Spinel
• Chromite
• Magnetite
• Plagioclase (in basalt)

In mantle xenoliths, it is associated with other ultramafic minerals.

Historical Discovery and Naming

The name “peridot” likely derives from the Arabic word faridat, meaning “gem.”

It was mined in ancient Egypt as early as 1500 BCE. Historically, peridot was sometimes confused with emerald due to its green color.

Peridot is also the birthstone for August in modern gemstone traditions.

Cultural and Economic Significance

Gemstone Use

Peridot is widely used in jewelry due to:

• Bright green color
• Good brilliance
• Moderate affordability

It is commonly cut into:

• Faceted gemstones
• Cabochons
• Beads

Fine-quality peridot with deep green color and minimal brown tones commands higher value.

Industrial Significance

Non-gem olivine is used in:

• Refractory materials
• Foundry sands
• Metallurgical processes

However, gem peridot is used primarily for jewelry.

Care, Handling, and Storage

Peridot requires moderate care.

Recommendations:

• Avoid strong impacts (can fracture)
• Protect from scratches by harder stones
• Avoid exposure to strong acids
• Clean with mild soap and water

Although reasonably durable, it is softer than quartz and can scratch with heavy wear.

Scientific Importance and Research

Peridot (olivine) is extremely important in Earth science:

• Major component of the Earth’s upper mantle
• Indicator of mantle composition
• Key mineral in igneous petrology
• Studied in volcanic and tectonic research

Its presence in mantle xenoliths provides direct evidence of Earth’s interior composition.

Extraterrestrial peridot in meteorites contributes to planetary science research.

Similar or Confusing Gemstones

Peridot may be confused with:

• Emerald (beryl)
• Green tourmaline
• Demantoid garnet
• Chrome diopside

Gemological testing (refractive index, birefringence, inclusions) helps distinguish peridot from other green gems.

Mineral in the Field vs. Faceted Gemstone

In the field, peridot appears as green crystals embedded in basalt or mantle xenoliths.

When faceted, it displays vibrant green brilliance and characteristic doubling of facet edges due to high birefringence.

In meteorites (pallasites), peridot may appear as transparent green crystals within metallic iron-nickel matrix.

Fossil or Biological Associations

Peridot forms through igneous processes deep within the Earth or in meteorites and has no biological origin.

Relevance to Mineralogy and Earth Science

Peridot is one of the most geologically significant gemstones because:

• Olivine dominates Earth’s upper mantle
• It records mantle temperature and pressure conditions
• It helps scientists understand tectonic processes

Its stability and transformation under high pressure are studied in mantle phase transition research.

Relevance for Lapidary, Jewelry, or Decoration

Peridot is well suited for jewelry due to:

• Hardness of 6.5–7
• Attractive green color
• Good brilliance

It is commonly used in:

• Rings (with protective settings)
• Earrings
• Pendants
• Bracelets

However, due to moderate hardness and brittleness, it is best suited for light to moderate wear.

Peridot remains a historically significant and scientifically important gemstone, valued both as a beautiful green gem and as a key mineral in understanding Earth’s mantle composition.