Obsidian
Mineraloid (volcanic glass) · SiO₂-rich glass
Obsidian is natural volcanic glass formed when lava cools too fast to crystallise, prized for its glassy black shine and razor-sharp edges.
What is obsidian?
Obsidian is a mineraloid, not a true mineral: it is volcanic glass formed when silica-rich lava cools so quickly that crystals never grow. It is usually jet-black and glassy, sometimes with a golden or rainbow sheen, or brown-and-black “mahogany” banding. Its conchoidal fracture produces extremely sharp edges, used for tools for millennia.
Properties
- Chemical formula
- SiO₂-rich glass
- Category
- Mineraloid (volcanic glass)
- Hardness (Mohs)
- 5–5.5
- Crystal system
- Amorphous
- Lustre
- Vitreous (glassy)
- Streak
- White
- Colour
- Black, mahogany, sheen varieties
- Cleavage / fracture
- None; conchoidal fracture
How to identify obsidian
- →Glassy, usually black, with curved shell-like (conchoidal) fracture.
- →Very sharp edges on broken pieces.
- →No crystals or grains visible (amorphous glass).
- →May show a golden or rainbow sheen, or mahogany banding.
Where obsidian is found
Obsidian forms at young volcanoes worldwide, with well-known sources in the USA (Oregon, California), Mexico, Iceland and Armenia.
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