Olivine (Peridot)
Olivine species and its gem variety peridot with properties, characteristic inclusions, sources, and identification.
Introduction
Olivine is an iron-magnesium nesosilicate ((Mg,Fe)₂SiO₄) crystallising in the
orthorhombic system; its gem variety is peridot, one of the few gemstones occurring in
only a single colour family. RI 1.654–1.690 (biaxial positive) with birefringence
0.035–0.038 causes visible back-facet doubling under a loupe, a reliable feature
absent in singly-refractive simulants such as demantoid garnet. SG 3.27–3.37, hardness
6.5–7 Mohs. [1] Colour is entirely iron-driven (Fe²⁺) with no heat treatment
available to alter it [2]; vivid lime-green without yellow or
brown is ideal. The three-line iron absorption spectrum at 493, 473, and 453 nm is
characteristic, as are "lily pad" inclusions (disc-shaped stress fractures around
chromite crystals), virtually diagnostic for peridot. The historic source at Zabargad
Island (St. John's Island) in the Red Sea supplied peridot for over 3,500 years; today
Pakistan's Kohistan district and China's Changbai Mountains are the principal producers,
with Myanmar's Mogok valley yielding the finest colour.
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 [1]
- 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⁺) [2]
- 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
High Birefringence Effect
Identification Summary
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
References
- ↑ 1. Read, P. (2008). Gemmology (3rd ed.). Butterworth-Heinemann. ISBN: 978-0-7506-6449-3. DOI: 10.4324/9780080507224.
- ↑ 2. Fritsch, E.; Rossman, G. (1987). An Update on Color in Gems. Part 1: Introduction and Colors Caused by Dispersed Metal Ions. Gems & Gemology, 23(3), 126–139. DOI: 10.5741/gems.23.3.126.