peridot

Peridot
General
Category	Mineral
Chemical formula	(Mg, Fe)2SiO4
Identification
Color	Yellow, to yellow-green, olive-green, to brownish
Crystal system	Orthorhombic
Cleavage	Poor
Fracture	Conchoidal
Mohs scale hardness	6.5-7
Luster	Vitreous (glassy)
Streak	White
Specific gravity	3.2-4.3
Refractive index	1.64-1.70
Birefringence	+0.036

Peridot (pronounced /ˈpɛrɪdɒt/ or /ˈpɛrɪdoʊ/) is gem-quality forsteritic olivine. The chemical composition of peridot is (Mg, Fe)₂SiO₄, with Mg in greater quantities than Fe.

The origin of the name "peridot" is uncertain. The Oxford English Dictionary suggests an alteration of Anglo-Norman pedoretés (classical Latin paederot-), a kind of opal, rather than the Arabic word faridat, meaning "gem".

Olivine in general is a very abundant mineral, but gem quality peridot is rather rare.

Peridot is one of the few gemstones that occur in only one color: basically an olive green. The intensity and tint of the green however depends on how much iron is contained in the crystal structure, so the color of individual peridot gems can vary from yellow-green through olive green to brownish green. The most valuable is considered a dark-olive green color.

Peridot crystals have been collected from some Pallasite meteorites. A famous Pallasite was offered for auction in April 2008 with a requested price of close to $ 3 million at Bonhams, but remained unsold.^[1] Peridot is the only gemstone found in meteorites.

Peridot olivine is the birthstone for August. It is sometimes mistaken for emeralds and other green gems. In fact notable gemologist George Frederick Kunz ^[2] discussed the confusion between emeralds and peridots in many church treasures, notably the "Three Magi" treasure in the Dom of Cologne, Germany.

Occurrence

Olivine, of which peridot is a type, is a common mineral in mafic and ultramafic rocks, and it is often found in lavas and in peridotite xenoliths of the mantle, which lavas carry to the surface; but gem quality peridot only occurs in a fraction of these settings.

Peridot olivine is mined in North Carolina, Arizona on the San Carlos Reservation, Hawaii, Nevada, and New Mexico, in the US; and in Australia, Brazil, China, Kenya, Mexico, Myanmar (Burma), Norway, Pakistan, Saudi Arabia, South Africa, Sri Lanka, and Tanzania.

In much antique jewelry, peridot could have come from Egypt: in the late 18th/early 19th century, peridot was taken from Egyptian ecclestial and other ornaments and reused in jewelry. Furthermore a location in Egypt was (re-) discovered but its location remains unknown.^[3]

The largest cut peridot olivine is a 310 carat (62 g) specimen in the Smithsonian Museum in Washington, D.C..

Peridot olivine with minor pyroxene, on vesicular basalt. (field of view = 35 mm)

Peridot from the San Carlos Apache reservation in Arizona.

Olive Green Peridot

Peridot with milky inclusions

References

External links

Wikimedia Commons has media related to: Peridot

• Gemstone.org Peridot
• USGS peridot data
• Emporia Edu
• Florida State University - Peridot