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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
Fracture: Large conchoidal
Dispersion: Very low (.01)
Heat sensitivity: Moderate
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.