Gem/Gem Topic of the Month

Each month this section will feature either a topic of interest to gem lovers or one special gemstone with background on the material and its value.

Go to: Homepage -- what's new in faceted gems -- what's new in designer cabochons and gem carvings -- gem of the month -- gem of the month archive -- birthstone of the month -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- about the ACS cutters -- settings for these gems --faceting information -- purchase UltraTec equipment

 

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 March, 1998 (Revised, August, 2004)

Peridot

To my mind, peridot is an under-appreciated gemstone. Perhaps this has become the case due to the public's familiarity with low quality, olivey material which is inadequately cut and polished. Admittedly it can look pretty awful, but the lime and apple green stones given custom cuts and polishes are something else again.

 

Peridot belongs to the forsterite-fayalite mineral series which is part of the olivine group. It is one of the "idiochromatic" gems, meaning that its color comes from the basic chemical composition of the mineral itself, not from minor impurities, and therefore it will only be found in shades of green. Historically important sources in Egypt have been superceded by today's main sources in Arizona and Pakistan and most recently China.

The high birefingence of this gemstone necessitates careful orientation in cutting to prevent "fuzziness" of facet reflections through the table. Distinctive, disk-like liquid and gas "lily pad" inclusions can often be seen under magnificatiion.

Peridot is the birstone for the month of August. Peridot is one of the gems which can be assumed to be natural and which is hard and tough enough for most jewelry uses, although use in rings without protective mountings should be limited to occasional wear or gently used pieces.


Value

The vast majority of peridot rough produces sub-carat sized stones which are ubiquitous in commercial quality jewelry and are quite inexpensive. Stones in the 1-4 carat range that have custom cuts and lack olive tones are much more highly valued and stones over 4 carats with good clarity, cut and color bring the highest prices of all.


Gemological Data

Makeup: an iron, magnesium, silicate

Luster: Sub-Vitreous to Vitreous

Hardness: 6.5 - 7

Crystal structure: Orthorhombic

Fracture: conchoidal

Density: 3.34

RI: 1.65 - 1.69

Birefringence: .036

All text and images, unless otherwise designated, © 2004 Barbara Smigel

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{Search our Catalog}

Go to: Homepage -- what's new in faceted gems -- what's new in designer cabochons and gem carvings -- gem of the month -- gem of the month archive -- birthstone of the month -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- about the ACS cutters -- settings for these gems --faceting information -- purchase UltraTec equipment