Lapidarium
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Obsidian

Volcanic glass, sharper than any steel blade.

Family
Volcanic glass
Formula
SiO₂ (70 to 75%) with Al, Na, K, Fe
Mohs
5 to 5.5
System
Amorphous

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Science

Obsidian is lava that cooled too fast to crystallise. Having no crystal structure it breaks with a conchoidal fracture into edges only a few nanometres thick, which is why it was the first cutting tool and is still used for some surgical scalpels. Colours come from magnetite (black), hematite (mahogany) and cristobalite spherulites (snowflake).

Mineral
Volcanic glass
Formula
SiO₂ (70 to 75%) with Al, Na, K, Fe
Crystal system
Amorphous
Mohs hardness
5 to 5.5
Specific gravity
2.35 to 2.45
Refractive index
1.480 to 1.510
Lustre
Vitreous
Colours
Black, Mahogany, Snowflake, Rainbow sheen

Composition

SiO₂ (70 to 75%) with Al, Na, K, Fe. Made of Silicon, Oxygen, Aluminium, Sodium, Potassium, Iron.

in formula

Origin

  • Mexico
  • United States
    Oregon, California
  • Iceland
  • Italy
    Lipari
  • Turkey
  • Ethiopia
  • Japan

Filled dots are primary sources

Rarity

Abundant
1 of 5

Abundant wherever rhyolite lava cooled fast. Rainbow and fire obsidian with thin-film colour are the scarcer forms.

  1. 1 Abundant
  2. 2 Common
  3. 3 Uncommon
  4. 4 Rare
  5. 5 Exceptional

Hardness

Obsidian sits at 5 to 5.5 on the Mohs scale. Glass and a knife blade are about as hard; it wears well with care.

Fingernail
Copper coin
Knife, glass
Steel file
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5
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10

Energy

Associations from crystal-healing tradition, recorded here as folklore. They are not medical claims.

Chakras

  • Root

Element

  • Earth

Zodiac

  • Scorpio
  • Sagittarius
  • Capricorn

Planets

  • Saturn
  • Pluto

Properties

  • Protection
  • Truth
  • Shadow work
  • Release
  • Grounding

Lore

Aztec warriors fought with the macuahuitl, a wooden club edged with obsidian blades, and priests used obsidian mirrors for divination; the god Tezcatlipoca is 'Smoking Mirror'. John Dee's scrying mirror in the British Museum is Aztec obsidian.

Its practice associations follow the blade: cutting through illusion, and confronting what is hidden.

  • Edge
    Fractured obsidian edges are about 3 nanometres, far finer than steel.
  • Dating
    Obsidian hydration rims let archaeologists date artefacts.