Opal

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.