igneous
Obsidian
Volcanic glass (SiO2 - Silicon Dioxide with impurities)
Hardness: 5-5.5 (Mohs); Color: Typically black, can be grey, green, or mahogany; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Typically black, can be grey, green, or mahogany
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 (Mohs); Color: Typically black, can be grey, green, or mahogany; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed from rapid cooling of felsic lava, preventing crystal growth. Usually found in margin areas of flows or at the edge of volcanic domes. Geological age varies but usually young (Cenozoic) because glass eventually devitrifies into crystals.
Uses & applications
Used for making jewelry (cabochons), scalpel blades (it can be sharper than steel), and ancient tools such as arrowheads and mirrors.
Geological facts
Obsidian is technically not a mineral because it lacks a crystalline structure; it is a 'mineraloid'. Ancient Aztec civilizations used obsidian for ceremonial 'macuahuitl' swords.
Field identification & locations
Identify by its characteristic conchoidal (shell-like) fracture and sharp edges. Common in volcanic regions like the American Northwest (Oregon/California), Mexico, and Iceland. Collectors look for 'Snowflake' or 'Rainbow' variants.
