Peridot is the gem-quality variety of the mineral olivine, treasured for its distinct green hue. It is one of the few gemstones that occurs in only a single color, though shades range from vibrant lime to deep olive green. Historically, it was associated with light and sometimes called the “evening emerald” because its color remains brilliant even under artificial light.
The Core Chemical Structure
Peridot’s identity is defined by its chemical formula, a magnesium iron silicate expressed as (Mg,Fe)2SiO4. This formula places the mineral in the olivine group, where magnesium (Mg) and iron (Fe) can substitute for one another within the crystal lattice. This substitution links peridot to a chemical spectrum known as a solid solution series.
This series has two endmembers: forsterite (Mg2SiO4), the pure magnesium silicate, and fayalite (Fe2SiO4), the pure iron silicate. Gem-quality peridot is generally closer to the magnesium-rich forsterite side of this spectrum. However, the iron content acts as the chromophore, giving peridot its characteristic yellowish-green to olive color. The finest green stones typically have an iron content below 15%.
Physical and Optical Properties
The mineral crystallizes in the orthorhombic system, meaning its structure is based on three axes of different lengths that intersect at right angles. This specific structure is responsible for a unique optical effect known as double refraction, or birefringence. When light passes through a well-cut peridot, the light ray is split into two, causing a visible doubling of the facet edges when viewed through the stone.
Peridot has a hardness ranging from 6.5 to 7 on the Mohs scale, which measures scratch resistance. This moderate hardness makes it suitable for jewelry, though it requires more care than harder gems like sapphire or diamond.
Geological Formation and Source
Peridot is unique among gemstones because it does not form in the Earth’s crust; instead, it crystallizes deep within the upper mantle, alongside diamond. This formation occurs at depths greater than 25 miles, in environments rich in iron and magnesium. The stones are then brought to the surface through rapid volcanic activity, often embedded in basaltic lava flows as rock fragments called xenoliths.
Major terrestrial deposits are found in volcanic regions. Notable mining locations include the San Carlos Apache Indian Reservation in Arizona, a principal source, and sites in Pakistan and Myanmar.
Peridot is also one of the few gemstones with an extraterrestrial source. Peridot crystals are a primary component of pallasite meteorites, a rare type of stony-iron meteorite. These cosmic origins link peridot to the early formation of the solar system, where the crystals formed in the mantle boundary of small planetary bodies. Pallasitic peridot differs slightly from its terrestrial counterpart, for instance, by lacking nickel traces sometimes present in volcanic peridot.