Olivine (Peridot)
Olivine species and its gem variety peridot with properties, characteristic inclusions, sources, and identification.
Introduction
Olivine ((Mg,Fe)₂SiO₄) is an iron-magnesium silicate, and its gem-quality
variety is known as peridot. One of the few gems that occur in only one
colour, peridot has been treasured since antiquity for its distinctive
yellow-green to olive green colour.
Peridot is the birthstone for August and has been found in meteorites,
making it one of the few extraterrestrial gems.
Mineralogy
Crystal System and Structure
- Crystal system: Orthorhombic
- Chemical formula: (Mg,Fe)₂SiO₄
- Series: Forsterite (Mg) to Fayalite (Fe)
- Gem composition: Usually ~88-92% forsterite
- Cleavage: Poor (rarely seen)
Physical Properties
| Property | Value |
|---|---|
| Hardness | 6.5-7 Mohs |
| Specific gravity | 3.27-3.37 |
| Refractive index | 1.654-1.690 |
| Birefringence | 0.035-0.038 (high) |
| Optic character | Biaxial positive |
| Pleochroism | Weak (yellowish-green to green) |
| Lustre | Vitreous to oily |
Colour
Peridot occurs in a single hue family:
- Chromophore: Iron (Fe²⁺)
- Colour range: Yellow-green to olive green to brownish-green
- Ideal: Vivid lime green without brown
- No treatment: Peridot is never heat-treated for colour
The finest colour is a vivid lime green without yellow or brown modifiers.
Iron content determines the depth of colour.
Lily Pad Inclusions
Major Sources
| Origin | Characteristics |
|---|---|
| Myanmar (Mogok) | Finest colour; limited availability |
| Pakistan (Kashmir) | Large, fine crystals |
| China (Changbai) | Major current producer |
| Arizona (San Carlos) | Smaller stones; Native American jewellery |
| Zabargad (Egypt) | Historic source; now exhausted |
| Vietnam | Good quality material |
| Ethiopia | Emerging source |
Zabargad Island
Characteristic Inclusions
Peridot shows distinctive internal features:
- Lily pads: Stress discs around chromite crystals (diagnostic)
- Chromite crystals: Small black octahedra
- Needle inclusions: Fine, oriented needles
- Negative crystals: Angular voids
- Healing feathers: Plane fractures with fluid
High Birefringence Effect
Identification Summary
Key features for peridot identification:
- RI: 1.654-1.690 (distinctive range)
- SG: 3.34 (characteristic)
- Birefringence: 0.035-0.038 (high; visible doubling)
- Colour: Yellow-green to olive green only
- Spectrum: Three Fe bands at 493, 473, 453nm
- Inclusions: Lily pads diagnostic
Distinguishing Similar Gems
| Gem | Key Distinction from Peridot |
|---|---|
| Demantoid garnet | Garnet is SR (no doubling); higher RI |
| Green tourmaline | Different RI range; different spectrum |
| Chrome diopside | Lower birefringence; different RI |
| Green zircon | Higher RI (1.93+); higher SG |
| Moldavite | Much lower RI (1.48); glass |
Care and Durability
Peridot requires some care in jewellery:
- Hardness 6.5-7 means it can scratch
- Sensitive to acids (even weak ones)
- Avoid ultrasonic and steam cleaning
- Warm soapy water is safe
- Best for earrings, pendants, occasional ring wear
- Protect from hard impacts