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Obsidian (volcanic siliceous glass)

Type: Apache tear

Color: Dark gray to brownish-gray. May be transparent, translucent, or nearly opaque.

Moh’s hardness: 5.5

Refractive index: 1.4-1.6

Critical angle: 42.5°

Specific gravity: 2.3-2.6

Cleavage: None

Fracture: Large conchoidal

Dispersion: Very low (.01)

Heat sensitivity: Moderate

Pleochroism: None

Birefringence: None

Crystal structure: Amorphous

Cutting: Beware that Apache tears may have internal pressures that can be released during sawing or cutting, causing the material to literally fly apart.  Therefore, eye protection is suggested.  Apache tears also tend to contain bubbles.  Large stones are usually too dark unless cut from the lightest material.

Polishing: Cerium oxide on most common polishing laps.

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