Peridot

For other uses, see Peridot (disambiguation).

Peridot
General
Category	Mineral
Formula (repeating unit)	(Mg, Fe)2SiO4
Identification
Color	Yellow, to yellow-green, olive-green, to brownish, sometimes a lime-green, to emerald-ish hue
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 (pron.: /ˈpɛrɨdoʊ/ or /ˈpɛrɨdɒt/) is gem-quality olivine. Olivine is a silicate mineral with formula of (Mg, Fe)₂SiO₄. As peridot is the magnesium-rich variety (forsterite) the formula approaches Mg₂SiO₄.

Etymology

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

The online Merriam Webster, however, says "French péridot, from Old French peritot First Known Use: circa 1706".

The Middle English Dictionary defines peridot with several variations : peridod, peritot, pelidod and pilidod -- in the examples other variants substitute y for i. (peridod, peritot, pelidod and pelido are also used in Old French). In Medieval Latin peridotus ,pl peridota.

The earliest use in England is in the register of the St Albans Abbey, in latin, and its translation in 1705 is possibly the first use of Peridot in English. It records that on his death in 1245 Bishop John bequeathed various items to the Abbey: "He gave also three noble Rings, in one whereof is an Oriental Sapphire, of wonderful bigness : In another is the Stone call'd Peridot, in the middle whereof is set a Sapphire of great beauty; it is said to be good against the Cramp, and is made almost in the form of a Buckler: in the third Ring is also an Oriental Sapphire, but less than the former."

A copy by Johannis de Trokelowe of the latin reads: Dedit etiam tres nobiles aureos annulos, in quorum uno habetur sapphirus orientalis mirae magnitudinis; in alio, lapis qui dicitur "peridotus," sive "pederotes;" in cujus medio sapphirus excellentis pulchritudinis collocatur, et habet virtutem spasmum potenter refrenandi. Iste lapis ad modum clypei fere formatur. In tertio vero annulo alius sapphirus includitur orientalis, sed minor quam prior de quo locuti sumus.

(Modified Google translate: He also gave three noble gold rings, the Oriental sapphire in one of which a surprisingly large, in another, the stone which is said to be a "peridotus," or "pederotes;" in the midst of which is placed a sapphire of surpassing beauty, and has the power of powerfully restraining convulsions. This stone is shaped almost like a shield. In the third case, an Oriental sapphire is included with another ring, but it is smaller than the first of which we have spoken.)

Other variants quoted by the Middle English Dictionary range from 1300 to 1500.

There are also early uses in French : The 1667 Le mercure indien, Ou Le Tresor Des Indes: Dans laquelle est ... - Page 87 by Pierre de Rosnel has two short chapters on Peridot. The 1694 "Nouveau Dictionnaire françois, Volume 2 By Pierre Richelet", in part reads : "PERIDOT, Sorte de pierre précieuse qui n'est pas fort considérable, qui tire fur une couleur qui rient du vert ..." or "Kind of gemstone that is not very considerable, which takes a color as laughing green ... " (Also a 1680 edition with a less colorful description)

Appearance

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 how much iron is contained in the crystal structure, so the color of individual peridot gems can vary from yellow- to olive- to brownish-green. The most valued color is a dark olive-green.

Occurrence

Geologically

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 can be also found in meteorites.

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

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

In meteorites

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.

Gemology

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

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

Peridot olivine is the birthstone for the month of August.

Gallery

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

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  Peridot from the San Carlos Apache reservation in Arizona.

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  Olive Green Peridot

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  Peridot with milky inclusions

External links

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