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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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