Obsidian
SiO₂ + impurities (amorphous) · Amorphous (no crystal structure) · also called volcanic glass, apache tears, snowflake obsidian
Obsidian is lava that cooled too quickly to crystallise, making it a natural glass rather than a true mineral. Its fracture produces edges finer than a surgical scalpel, which is why it was prized for tools and blades for tens of thousands of years.
Properties
- Mohs hardness
- 5–5.5
- Streak
- white
- Lustre
- vitreous
- Colour
- Usually jet black. Brown 'mahogany' and gold- or rainbow-sheened varieties come from tiny aligned inclusions.
- Cleavage
- No cleavage — it is glass, with no internal crystal structure at all.
- Fracture
- Conchoidal, exceptionally sharp
- Specific gravity
- 2.35–2.6 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm obsidian
- Curved, glossy, razor-sharp fracture surfaces. Handle freshly broken pieces carefully.
- Thin edges are translucent brown when held to a strong light, even on jet-black material.
- Softer than quartz — a quartz crystal scratches it.
What obsidian gets confused with
Each of these is settled by one test. Run that test first.
Industrial slag is often bubbly and can be lighter or heavier; obsidian is dense and bubble-poor.
Jet is fossil wood — far lighter, warmer to the touch, and it burns.
Chalcedony is waxy and harder; obsidian is glassy and softer.
Where it is found
Rhyolitic volcanic areas — the western United States, Iceland, Mexico, Turkey, and the Mediterranean islands.
Is it worth anything?
Very inexpensive. Sheen and rainbow varieties are cut for jewellery but remain affordable.