Peridot is a recognized gemstone, highly valued for its distinctive green hue. It serves as the modern birthstone for August, with a lineage that stretches back to ancient civilizations. Its unique color ranges from a light yellowish-green to a rich, deep olive shade. Peridot’s story is remarkable because its origins are not limited to Earth, distinguishing it from most other popular jewelry stones.
Peridot’s Mineralogical Classification
The stone known as peridot is the gem-quality expression of the mineral olivine, a magnesium iron silicate. Its chemical formula is (Mg, Fe)\(_2\)SiO\(_4\), indicating that magnesium and iron are integral components. Olivine is an abundant mineral found deep within the Earth’s mantle, but only a fraction is classified as the gemstone peridot. The presence of iron within the crystal lattice is the sole cause of peridot’s green coloration, classifying it as an idiochromatic gem.
The ratio between magnesium and iron determines the exact shade, with higher iron content yielding a deeper green color. Gem-quality peridot must exhibit sufficient transparency, clarity, and color saturation. Most olivine crystals are small and heavily weathered when found, making the large, pure crystals required for faceting relatively uncommon.
Defining Characteristics of Gem Quality
Peridot is primarily prized for its characteristic green color, which can vary from vibrant lime green to brownish-green. The most sought-after hues are a pure, saturated green or a slight yellowish-green, typically resulting from an iron content of 12 to 15 percent. This inherent color does not change under artificial light, a property that historically led the Romans to refer to it as the “emerald of the evening.”
The gem’s durability is rated between 6.5 and 7 on the Mohs scale of hardness. This moderate hardness means peridot is suitable for various jewelry settings, but it requires more care than harder stones like sapphire or diamond to avoid scratching. Peridot exhibits a strong optical property known as birefringence, meaning a light ray entering the crystal is split into two. This causes a visible doubling of the back facets, a distinct feature used by gemologists to identify peridot. The stone also displays a vitreous luster and a relatively high refractive index, which contributes to its brilliance and sparkle.
Sources and Notable Discoveries
Peridot forms under extreme conditions, crystallizing deep within the Earth’s upper mantle before being brought to the surface by volcanic activity. This formation process is shared by only one other gemstone, the diamond, making peridot an “extreme” gem.
Historically, the most famous source was Zabargad Island, now known as St. John’s Island, located in the Red Sea off the coast of Egypt, where mining began as early as 300 BCE. Today, major terrestrial sources include the San Carlos Apache Indian Reservation in Arizona, which is a significant producer of commercial-grade material. Fine quality stones, often displaying the richer, deep olive-green color, are sourced from deposits in Pakistan and Myanmar.
Peridot has a unique origin among gemstones because it is also found in pallasite meteorites, which are remnants of the early solar system. These extraterrestrial crystals, embedded in an iron-nickel matrix, confirm peridot’s presence beyond our planet and offer a rare glimpse into the materials that formed our solar system.