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
Fracture: Conchoidal, brittle, splintery
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.