Lapidarium
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Opal

Silica spheres and water, stacked so neatly they diffract light into colour.

Family
Opal
Formula
SiO₂·nH₂O
Mohs
5.5 to 6.5
System
Amorphous

Science

Opal is hydrated silica with 3 to 10 percent water, deposited from groundwater in cracks and around fossils. In precious opal the silica spheres are all the same size, 150 to 300 nanometres, and stack in a lattice that diffracts light: the play-of-colour changes with sphere size and viewing angle. Ethiopian opal is hydrophane, absorbing water and going transparent.

Mineral
Opal
Formula
SiO₂·nH₂O
Crystal system
Amorphous
Mohs hardness
5.5 to 6.5
Specific gravity
1.98 to 2.25
Refractive index
1.370 to 1.470
Lustre
Vitreous to waxy
Colours
White, Black, Fire orange, Play-of-colour
Birthstone
October

Composition

SiO₂·nH₂O. Made of Silicon, Oxygen, Hydrogen.

in formula

Origin

  • Australia
    Lightning Ridge, Coober Pedy
  • Ethiopia
    Wollo, since 2008
  • Mexico
    Querétaro, fire opal
  • Brazil
  • United States
    Virgin Valley, Nevada
  • Peru

Filled dots are primary sources

Rarity

Uncommon
3 of 5

Common opal is ordinary; precious opal with play-of-colour is scarce, and black opal from Lightning Ridge is among the rarest gems by value.

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

Hardness

Opal sits at 5.5 to 6.5 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
1
2
3
4
5
6
7
8
9
10

Energy

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

Chakras

  • Crown
  • Sacral

Element

  • Water

Zodiac

  • Libra
  • Scorpio
  • Cancer
  • Pisces

Planets

  • Moon
  • Venus

Properties

  • Creativity
  • Emotional expression
  • Inspiration
  • Amplification

Lore

Romans ranked opal above all gems; the senator Nonius chose exile over surrendering his to Mark Antony. Its bad-luck reputation dates only to Walter Scott's novel Anne of Geierstein in 1829, which halved the opal market within a year.

Aboriginal tradition says the Creator came to Earth on a rainbow and the stones where he stepped came alive with colour.

  • Play-of-colour
    Diffraction from ordered silica spheres, not pigment.
  • Mars
    Opal deposits have been detected on Mars, evidence of past water.