Natural Glass – Obsidian

Description:

once molten natural materials; amorphous in structure; chemical composition variable, but mostly silica; the types cut as gems include: obsidian (natural glass of volcanic origin).

Appearance:

• transparent to opaque, sometimes with sheen 
• gray to black (often spotted with white), brown to brownish yellow, orange, or red;rarely green, blue, purple
• phenomena: iridescence, chatoyancy (rare)

Variety and Trade Names:

• banded obsidian – obsidian showing curved or sinuous bands 
• onyx obsidian – obsidian showing straight, parallel bands 
• sheen obsidian – typically has a silvery or golden looking sheen 
• snowflake obsidian – white patches in a black or dark gray background; also call flowering obsidian 
• rainbow obsidian – a variety of obsidian with iridescent colors

Identification:

Refractive Index: 1.49(+.020, -.010)
Gravity: obsidian – 2.40 (+.10, -.07)
Obsidian – gas bubbles, crystallites, stubby needle-like inclusions, may be banded or have numerous oriented inclusions that cause a sheen
Hardness: 5 to 51/2 ; Toughness: fair to good Stability

Stone Information from:

Gem Reference Guide, by GIA, ISBN 0-87311-019-6
Gemstones of the world, Third Edition, by Walter Schumann, ISBN 1-4027-4016-6
https://www.gemdat.org/gem-8519.html
https://www.mindat.org/min-8519.html