Gem library
Obsidian
Volcanic glass, sharper than steel
Obsidian is not a mineral but a glass: rhyolitic lava chilled so abruptly that its atoms had no time to organise into crystals. Its fracture produces the finest edges known — sharper than a steel scalpel.
- Hardness (Mohs)
- 5 – 5.5
- Density
- 2.35 – 2.60
- Crystal system
- Amorphous — no crystal structure
- Refractive index
- 1.48 – 1.51
- Cleavage
- None
Mineralogical identity
- Family
- Vitreous volcanic rock (non-crystalline)
- Species
- Natural glass
- Variety
- Obsidian
- Chemical formula
- SiO₂ ~70 % + Al, Na, K (rhyolitic composition)
- Crystal system
- Amorphous — no crystal structure
- Classification
- Rock
- Rarity
- Very common
- Indicative price
- €0.20 – 3/ct
Etymology
After Obsius or Obsidius, a Roman to whom Pliny attributes the discovery of a similar stone in Ethiopia.
Physical & optical properties
- Hardness (Mohs)
- 5 – 5.5
- Density
- 2.35 – 2.60
- Refractive index
- 1.48 – 1.51
- Birefringence
- None (isotropic)
- Cleavage
- None
- Fracture
- Perfect conchoidal, extremely sharp edges
- Lustre
- Vitreous to greasy
- Diaphaneity
- Opaque, translucent in thin section
- UV fluorescence
- None
- Pleochroism
- None
- Colours
- black, brown, snowflake, rainbow, mahogany
Formation, geology & history
A glass born of quenching
When a viscous, silica-rich lava reaches open air or meets water, it freezes within minutes. No crystal structure has time to appear: the result is an amorphous solid, a natural glass. It is a geologically young and unstable material: over millions of years obsidian devitrifies, its atoms reorganise, and it turns into perlite or rhyolite. That is why almost no obsidian older than twenty million years exists. The varieties bear the trace of that nascent devitrification: snowflake obsidian owes its white rosettes to germinating cristobalite spherulites.
The world's finest blade
Obsidian's conchoidal fracture can produce an edge a few nanometres thick, against several hundred for a sharpened steel scalpel. Neolithic cultures in Anatolia, Mexico and the Pacific made it their strategic raw material: the blades of Çatalhöyük, the Aztec macuahuitl, ritual knives. The obsidian trade is one of the earliest documented long-distance exchange networks — its geochemical signature allows precise sourcing, which revolutionised archaeology. Some ophthalmic surgeons still use obsidian blades for incisions that heal exceptionally well.
Varieties and optical effects
Black obsidian is the commonest. Mahogany obsidian takes its brown-red from hematite. Mexican golden and rainbow obsidian owe their shimmer to aligned nanoparticles of magnetite that produce metallic iridescence, visible only at certain angles. Snowflake obsidian is scattered with white rosettes, and «Apache tears» are small rounded translucent nodules. Note: the electric blue, purple and neon green obsidians of the trade are coloured industrial glass.
How to recognise it
- Sharp conchoidal fractureA splinter of obsidian cuts skin at the slightest contact. The edge shows concentric shell-like ripples — the signature of glass.
- Hardness 5A steel knife scratches it and it does not scratch window glass. This is the best discriminator against onyx (hardness 7).
- Relative lightnessDensity around 2.4, appreciably lighter than onyx or black tourmaline for the same volume.
Imitations, treatments & confusions
- Coloured industrial glassAll bright blue, purple or neon green «obsidians» are manufactured glass. Natural obsidian does not exceed blacks, browns, greys and discreet iridescence.
- Foundry slagVitreous blast-furnace slags look very much like obsidian but contain aligned bubbles and sometimes metallic inclusions.
- Black resinLight, warm, scratched by a fingernail. The weight test in the hand and a dull sound on knocking give it away.
Deposits & origins
Mexico (Hidalgo, Jalisco — golden and rainbow obsidian), United States (Oregon, California: snowflake obsidian), Iceland, Italy (Lipari, Pantelleria), Turkey (central Anatolia, source of Neolithic tools), Armenia, Japan, Indonesia
Care & precautions
- Warm soapy water and a soft cloth; brief rinsing without long soaking.
- No salt: natural glass is slightly attacked by saline solutions in the long run.
- Fragile on impact despite its appearance: at hardness 5 with conchoidal fracture it chips easily. Store separately.
- Avoid ultrasonic cleaners and thermal shock, which can start a fracture.
Traditions & crystal healing
- Chakras
- Root
- Zodiac signs
- Scorpio · Sagittarius
- Element
- Fire · Earth
Tradition makes it a mirror: the Aztecs and Maya polished obsidian discs for divination, and the Elizabethan magus John Dee owned such a mirror, now in the British Museum. It is credited with a revealing role — confronting you with what you avoid looking at — and a cutting protective function, to be handled with measure.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Is obsidian a stone?
It is a rock, not a mineral: it has no crystal structure at all. Mineralogically it is a natural glass, which explains its fracture and modest hardness.
Why is there no very old obsidian?
Because glass is unstable: over tens of millions of years it devitrifies into rhyolite or perlite. All known obsidian is less than twenty million years old.
Does blue obsidian exist?
Not in a vivid natural form: the electric blues of the trade are industrial glass. The only natural bluish effects are discreet metallic iridescence from magnetite.