igneous
Obsidian
Obsidian (Volcanic Glass), primarily SiO2 (Silicon Dioxide)
Hardness: 5–5.5 on Mohs scale; Color: Black, deep brown, or greyish with occasional bands; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6
- Hardness
- 5–5
- Color
- Black, deep brown, or greyish with occasional bands
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–5.5 on Mohs scale; Color: Black, deep brown, or greyish with occasional bands; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6
Formation & geological history
Formed from the rapid cooling of silica-rich (rhyolitic) lava, which cools so quickly that mineral crystals do not have time to grow. Most specimens are geologically young (less than few million years old) because glass is unstable and devitrifies over time.
Uses & applications
Historically used for making sharp tools like arrowheads and knives; currently used in jewelry (cabochons), scalpel blades for delicate surgeries, and as a decorative stone in landscaping and collection.
Geological facts
Obsidian is often called 'nature's glass'. Despite its dark appearance, thin slivers are often translucent. Some varieties like 'Rainbow' or 'Sheen' obsidian contain microscopic mineral inclusions that create colorful iridescence.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture patterns and sharp edges. Common in volcanic regions such as the American West (Oregon, California), Mexico, Iceland, and Japan. Collectors should look for smoothness and lack of internal bubbles found in slag glass.
