Agates and Opals


  • finely crystalline, microcrystalline or cryptocrystalline quartz
  • termed chalcedony: often fibrous, botryoidal masses, fibres perpendicular to hummocks
  • S.G. ~ 2.6 (lower than quartz, slightly porus)
  • silica rich solutions fill up cavities in rocks to form geodes
  • porosity allows coloration by dyes:
  • Agate is most common variety of chalcedony
  • crystals at core may be amethyst note the color bands


  • it's made from essentially amorphous or non-crystalline material
  • despite the lack of crystal structure, opal does display internal order!
  • composition = SiO2*nH2O (up to 30 % water)
  • hardnesss = 5.5 -6.0
  • color = variable, with play of color
  • "opalescence" = color change with orientation

View some opal images!

Opal Varieties

There are three general opal groups:

Precious Opal

Precious opal, which displays 'opalescence' (spectral color, irridescence that changes with the angle at which the gem is viewed).

Two subgroups:

White opal is an opaque stone in which the colors appear as flashes or speckles.

Black opal contains fire with a dark body color. These are less common and tend to be costly.


  • precious opal note the rainbow-like iridescence.
  • colors and patterns
  • opalized wood
  • shell
  • stem

  • Fire Opal
    Fire opal is transparent or translucent with an orange or red body color. Fire opals are named for their color (but are not opalescent). The term is often misinterpreted to indicate that fire opal is opalescent because in precious opal, (with a play of color) the play itself is called fire.

     Examples of fire opal and related varietites

    (Mexican) water opal is transparent and contains flashes of fire.

  • jelly opal
  • also found in a rhyolite matrix
  • "water-bearing opal"

  • Water can still be seen inside the opal!
  • fire opal
  • note the orange color, no opalescence
  • Common Opal
    Common opal is rather opaque. Many names are used to describe varieties, e.g., honey opal, milk opal, moss opal, etc.


  • common opal: opaque, no play of color
  • "petrified" opal
  • Where is opal formed and where is it found?


    precipitated near the Earth's surface from circulating ground water or hydrothermal solutions; esp. where seasonal rainfall and extended dry periods, often as linings of cavities or crack fillings


    90 % of World's supply of opal from South Australia

    Assembled Opal Products

    Doublets and Triplets:
    Doublets and triplets consist of a thin slice of opal cemented to a dark substrate. In the case of opal triplets, a colorless glass or crystalline cap is cemented to the opal slice.

    Doublets and Triplets are common, as these are substantially less expensive than precious opal.


  • doublets
  • triplets
  • Enhancements

    • impregnation with plastic, surface oiling
    • carbon or sugar treatment:
      • heating the opal in paper (manure) leaves a deposit of carbon below the surface of the stone
      • soaking the opal in sugar-rich solutions in sulfuric acid bath gives it a peppery 'pinfire' effect (play of color)

      • (this is a superficial treatment, which leads to low S.G. and porus stones)

    Preservation and handing of opals

    Synthetic Opals

    There are several manufacturers of synthetic opals, including Gilson Opal, Inamori Opal, etc.


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