Peridot is gem-quality olivine. It is a silicate mineral with the formula of (Mg, Fe)2SiO4. As peridot is the magnesium-rich variety of olivine (forsterite). The formula approaches Mg2SiO4.
Peridot 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 percentage of iron in the crystal structure. So the color of individual peridot gems can vary from yellow, to olive, to brownish-green. In rare cases, it may occur in a medium-dark toned, visually pure green with no secondary yellow hue or brown mask.
Olivine in general is a very abundant mineral, but gem quality is rather rare. This is due to the mineral’s chemical instability on the Earth’s surface. We usually find olivine as small grains. And tends to exist in a heavily weathered state, unsuitable for decorative use. Large crystals of forsterite, the variety most often used to cut peridot gems, are rare; as a result olivine is considered to be precious
Peridot has a very long written history. Ancient papyri record the mining of these stones as early as 1500 BC. The main source of peridot in the ancient world was Topazo Island (now Zabargad or St. John’s Island) in the Egyptian Red Sea. In Ancient times, stones were also used for carved talismans. Island habitants were forced to collect the gems due to the Pharaoh’s treasury. Another legend says that jealous watchers who had orders to put to death any trespassers guarded the entire island. The story continues that the miners worked in the daytime as well as night. We can found the gems after nightfall due to their radiance. The miners would mark the spot at night for retrieval the following day.
Therefore, the largest cut is a 310 carat (62 g) specimen in the Smithsonian Museum in Washington, D.C.
COLOR: Yellowish green
REFRACTIVE INDEX: 1.65 to 1.69
BIREFRINGENCE: 0.035 to 0.038
SPECIFIC GRAVITY: 3.34
MOHS HARDNESS: 6.5 to 7
Peridot, from Mondulkiri, Cambodia