Geological Information

Geological information of Opal

• There are two main forms: the non-precious common opal and the internal fire flashing precious stone opal.

General Identification

• Type: Mineraloid
• Formula: SiO₂ · nH₂O
• Hardness: 5.5 - 6.5
• Lustre: Vitreous, Waxy, Greasy, Dull
• Crystal System: Amorphous
• Crystal habit: Irregular veins, in masses, in nodules
• Colour: Colourless, white, yellow, red, orange, green, brown, black, blue

• From Mindat.org - Opal is classified into four types:

• • - Opal-CT: contains cristobalite-tridymite
  • - Opal-C: contains cristobalite
  • - Opal-AG: amorphous (Amorphous-Gel)
    • closely packed amorphous silica spheres form a diffraction grating to create precious opal
  • Amorphous-Network (found as hyalite).

Special Notes

• It is absorbent and reflective.
• its water content may range from 3 to 21% by weight,
  • but is usually between 6 and 10%.
• Because of its amorphous character, it is classed as a mineraloid,
  • unlike crystalline forms of silica, which are classed as minerals.
• "Besides the gemstone varieties that show a play of color, the other kinds of common opal include the milk opal, milky bluish to greenish (which can sometimes be of gemstone quality); resin opal, which is honey-yellow with a resinous luster; wood opal, which is caused by the replacement of the organic material in wood with opal; menilite, which is brown or grey; hyalite, a colorless glass-clear opal sometimes called Muller's glass; geyserite, also called siliceous sinter, deposited around hot springs or geysers; and diatomaceous earth, the accumulations of diatom shells or tests. " Wikipedia
• The town of Coober Pedy in South Australia is a major source of opal.
• Can be delicate unless a "triplet" (top layer is quartz and bottom layer stone) so
  • do not wear precious stones in rings when places hands in chemical including washing-up liquid.
• There are imitation opals that can fool include synthetic opalite.
  • Most found in crystal shops are obviously not opal if you know what opal looks like.
• In the Middle Ages: considered a stone that could provide great luck because it was believed to possess all the virtues of each gemstone whose color was represented in the color spectrum of the opal.