Peridot

Peridot is the gem variety of olivine - one of the most common minerals in the Earth's upper mantle but relatively rare at the surface. Olivine crystallizes at extreme temperatures (1,200-1,500 degrees Celsius) deep in the mantle, and reaches the surface through two dramatic mechanisms.

The most common is volcanic eruption - basaltic magma carries olivine crystals upward from depth, depositing them in lava flows and volcanic bombs. The San Carlos deposit in Arizona produces peridot from weathered basalt flows. The second mechanism is tectonic uplift, where mantle rock is thrust to the surface along fault zones - the Kohistan deposit in Pakistan formed this way.

Peridot holds a unique distinction: it's one of the few gemstones found in meteorites. Pallasite meteorites contain olivine crystals embedded in an iron-nickel matrix - extraterrestrial peridot formed in the cores of destroyed protoplanets billions of years ago. These meteoritic specimens are among the most expensive peridot per carat.

The green color comes from iron (Fe²⁺) in the crystal structure. Unlike many gemstones that come in multiple colors, peridot is idiochromatic - its color comes from an essential element in its chemistry, not a trace impurity. Peridot is always green.

Peridot's distinctive yellow-green color and high double refraction are its best diagnostic features. When you look through a faceted peridot at an angle, you can see doubled facet edges (a result of the strong birefringence) - this is visible even without a loupe in larger stones.

At hardness 6.5, peridot is moderately durable. Distinguish from green tourmaline (darker green, different crystal shape), chrysoberyl (higher hardness, different luster), and green glass (no double refraction, may contain bubbles).