Opal Faceting Information

Covers essential opal faceting information and related issues, research, tips, and guides about Opal Faceting. Choose what best fits the criteria for Opal.

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By Jeff R. Graham

Opal Faceting Information

Color: Colorless, yellow, orange, red
Moh's Hardness: 5-6.5
Refractive Index: 1.44-1.46
Critical Angle: 43.98°
Specific Gravity: 1.9-2.23
Cleavage: None
Fracture: Conchoidal, brittle, splintery
Dispersion: None
Heat Sensitivity: High to moderate
Birefringence (double refraction): None
Crystal Structure: Amorphous

Treatments: Generally none. However it is very common practice for rough dealers to sell them in water. The Opal absorbs water and weighs more this way. The water will also hide flaws and cracks. Never buy Opal in water. Mexican opal should be "seasoned" by leaving it open to the air for at least a few months before cutting to assure stability.

What I prefer to polishing with: Cerium oxide on a Tin lap, or a Spectra Ultralap

Jeff's Comments: Cold dop… Use glue when dopping to prevent damage to the stone by heat. Stones that display opalescent fire are usually better to cut as cabochons.

Design Notes: Use Opal designs and some Quartz designs that have been adjusted for the lower refractive index of Opal.

Jeff R. Graham

The late Jeff Graham was a prolific faceter, creator of many original faceting designs, and the author of several highly-regarded instructional faceting books such as Gram Faceting Designs.

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Opal, 3.51 cts, 1.3 x 1.2 x 0.5 cm, cut by Spectrum Award-winning faceter Mark Kaufman. Tsehay Mewcha, Wegeltena, Delanta Woreda, South Welo, Amhara Region, Ethiopia. © Rob Lavinsky, <a href=https://www.gemsociety.org/article/opal-faceting-information/"https://www.irocks.com/">www.iRocks.com. Used with permission." src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" decoding="async" data-nimg="responsive" style="position:absolute;top:0;left:0;bottom:0;right:0;box-sizing:border-box;padding:0;border:none;margin:auto;display:block;width:0;height:0;min-width:100%;max-width:100%;min-height:100%;max-height:100%;object-fit:cover;object-position:50% 50%"/>

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Opal Faceting Information