Peridot
August's Birthstone

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CHEMISTRY Mg₂SiO₄-Fe₂SiO₄
CRYSTALLOGRAPHY Orthorhombic
REFRACTIVE INDEX 1.635 - 1.673
HARDNESS 7
SPECIFIC GRAVITY 3.3 - 3.4
CLEAVAGE Imperfect
HEAT SENSITIVE No

WEARABILITY* Very Good
SPECIAL CARE INSTRUCTIONS None

ENHANCEMENTS None

*Wearability is graded as Excellent, Very Good, Good, Poor, and Forget It!
For more details see the article on "Hardness and Wearability."

...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 its color comes from the basic chemical composition of the mineral itself, not from minor impurities, and therefore 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 Historically important sources in Egypt have been superceded by today's main sources in Arizona and Pakistan. The high birefringence of this gemstone necessitates careful orientation in cutting to prevent "fuzziness" of reflections through the table. A distinctive, disk-like liquid and gas "lily pad" inclusion can often be seen under magnification.

Value

... The vast majority of peridot rough produces sub-carat sized stones which in commercial quality are quite inexpensive. Both Sinkakas and Federman agree that stones in the 1- 4 carat range, or those with custom cuts, may command $40/ct. or more, while those over 4 carats increase exponentially in value to $100-150/ ct in largest sizes.

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Text and photos courtesy of Barbara Smigel at Artistic Colored Stones.

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