COLOR TYPES FOR PERIDOT

A brief discribtion of the color type for Peridot used at Agl Thai Laboratory Gemology.

Peridot: The Egyptians called it the “gem of the sun.” is gem-quality olivine and a silicate mineral with the formula of (Mg, Fe)₂SiO₄. As peridot is a magnesium-rich variety of olivine (forsterite), the formula approaches Mg₂SiO₄. Its green color is dependent on the iron contents within the structure of the gem. Peridot occurs in silica-deficient rocks such a volcanic basalt as well as in meteorites. Peridot is one of only two gems observed to be formed not in the Earth’s crust, but in molten rock of the upper mantle.

The intensity and tint of the unique green color depends on how much iron is contained in the crystal structure, so the shade of individual peridots can vary from slightly yellowish Green to Yellow- Green to brownish Green.

Hardness: 6.5 to 7

Cleavage: rarely seen; Imperfect to distinct in one direction

Specific Gravity: 3.34 (+ 0.14 – 0.07)

Refractive Index: 1.654–1.690 (±0.020).

Birefringence: 0.035 to 0.038

Optic Character: Biaxial (positive or negative; the beta index is usually near halfway between alpha and gamma)

Pleochroism: Weak to moderate, dichroic; noticeable in a dark stone

UV Fluorescence: Generally inert

Dispersion: 0.020