Peridot

Introduction

Peridot is a gem-quality variety of the mineral olivine, which is composed of magnesium iron silicate. It is one of the few gemstones that occur in only one color, an olive-green. The intensity and tint of the green, however, depends on the amount of iron present in the crystal structure. Peridot is the birthstone for the month of August and has been prized for its beauty and rarity since ancient times.

Mineralogical Characteristics

Peridot belongs to the forsterite-fayalite mineral series, with forsterite (Mg2SiO4) and fayalite (Fe2SiO4) being the end members. The gemstone's chemical formula is (Mg, Fe)2SiO4, indicating that magnesium and iron can substitute for each other within the crystal lattice. The presence of iron is responsible for the green color, with higher iron content resulting in a deeper green hue.

The crystal system of peridot is orthorhombic, and it typically forms in short, stubby prismatic crystals. It has a hardness of 6.5 to 7 on the Mohs scale, making it relatively durable but still susceptible to scratching. The specific gravity ranges from 3.2 to 4.3, and it has a vitreous to oily luster.

Geological Formation and Occurrence

Peridot is primarily found in mafic and ultramafic rocks, such as basalt and peridotite, which are rich in magnesium and iron. It can also be found in xenoliths, which are fragments of rock trapped in a larger rock during the latter's development and solidification. These xenoliths are often brought to the surface by volcanic activity.

Significant deposits of peridot are found in various locations around the world, including:

The San Carlos Apache Indian Reservation in Arizona, USA, which is one of the most productive sources of peridot.
The Zabargad Island (St. John's Island) in the Red Sea, Egypt, historically known for its high-quality peridot.
Myanmar (Burma), which produces some of the finest peridot gemstones.
China and Pakistan, which have also become important sources in recent years.

Historical and Cultural Significance

Peridot has been valued since ancient times. The Egyptians referred to it as the "gem of the sun" and believed it had protective powers. It was mined on Zabargad Island as early as 1500 BCE. The Romans called it "evening emerald" because its green color did not darken at night and remained visible by lamplight.

In medieval Europe, peridot was used to adorn churches and was believed to ward off evil spirits. It was also considered a symbol of purity and morality. The gemstone has been found in meteorites, specifically in pallasites, which are a type of stony-iron meteorite, adding to its allure and rarity.

Physical and Optical Properties

Peridot's optical properties are characterized by its high double refraction, which means that light entering the gemstone is split into two rays that travel at different speeds. This results in a doubling of the back facets when viewed through the table of the gemstone. This property can be used to distinguish peridot from other green gemstones, such as emerald.

The refractive index of peridot ranges from 1.65 to 1.69, and it has a birefringence of 0.036 to 0.038. The gemstone exhibits strong pleochroism, showing different shades of green when viewed from different angles. It also has a distinct absorption spectrum, with lines at 493, 473, and 453 nm, which can be observed using a spectroscope.

Synthetic and Imitation Peridot

Synthetic peridot is not commonly produced, as the natural gemstone is relatively abundant and affordable. However, there are several imitation materials that can be mistaken for peridot, including green glass, synthetic spinel, and cubic zirconia. These imitations can often be identified by their physical and optical properties, such as their refractive index and specific gravity, which differ from those of natural peridot.

Gemological Identification

Gemologists use a variety of techniques to identify peridot and distinguish it from other green gemstones. These techniques include:

**Microscopic examination**: Peridot often contains characteristic inclusions, such as lily pad inclusions, which are disk-shaped inclusions that resemble lily pads.
**Refractometer**: Measuring the refractive index can help identify peridot, as it falls within a specific range.
**Spectroscope**: Observing the absorption spectrum can reveal the presence of iron, which is indicative of peridot.
**Specific gravity**: Determining the specific gravity can help differentiate peridot from other gemstones with similar appearances.

Cutting and Polishing

Peridot is typically cut into faceted gemstones to enhance its brilliance and color. The most common cuts include oval, round, and cushion shapes. Due to its relatively low hardness, care must be taken during the cutting and polishing process to avoid chipping and scratching. Polishing is usually done with diamond or corundum abrasives to achieve a high-quality finish.

Uses in Jewelry

Peridot is a popular choice for various types of jewelry, including rings, earrings, necklaces, and bracelets. Its vibrant green color makes it an attractive option for both casual and formal wear. However, due to its moderate hardness, peridot jewelry should be worn with care to avoid damage. It is often set in protective settings, such as bezel or channel settings, to minimize the risk of scratching.

Care and Maintenance

To maintain the beauty of peridot jewelry, it is important to follow proper care and maintenance guidelines:

**Cleaning**: Peridot can be cleaned using warm, soapy water and a soft brush. Ultrasonic cleaners and steam cleaners should be avoided, as they can damage the gemstone.
**Storage**: Peridot should be stored separately from other gemstones and jewelry to prevent scratching. Soft pouches or lined jewelry boxes are recommended.
**Avoiding exposure**: Peridot should be kept away from harsh chemicals, extreme temperatures, and prolonged exposure to sunlight, as these can affect its color and durability.

Metaphysical Properties

While scientific evidence does not support the metaphysical properties attributed to gemstones, peridot has been associated with various beliefs and practices throughout history. It is thought to bring prosperity, happiness, and emotional balance. Some believe that peridot can help alleviate stress, enhance creativity, and promote healing.

Conclusion

Peridot is a fascinating gemstone with a rich history and unique mineralogical properties. Its vibrant green color, combined with its relative affordability, makes it a popular choice for jewelry and collectors. Understanding the characteristics, formation, and care of peridot can enhance one's appreciation for this beautiful gemstone.