Opal

Common opal found near Greenwater, WA

WHAT IS IT?

  • A hydrated amorphous form of Silica — in other words, water containing non-crystalline silicon dioxide (what quartz is made of)

  • Consists of similar sized microscopic silica arranged in a cubic closet packed structure. Trace elements (for colored stone) and water (3% to 21%)

  • Deposited at low temperatures (usually less than 200 degrees Celsius) when feldspar. alumina, and other minerals that are found in “opal clay” come in contact with water and electrolytes

  • Hardness: 4.5 to 6.5 (harder than fluorite, softer than quartz)

  • The birthstone for October

HISTORY

  • The name is derived from the Sanskrit word "upala". meaning "valuable stone"

  • First used 4000 years ago

  • The ancient Greeks believed opal were the tears of Zeus and gave one the gift of prophecy

  • Australia Aborigine legend says they formed when the creator's foot touched the base of a rainbow

  • Ancient Romans wore opal as a symbol of hope and purity and as a cure for illness (especially Of the eyes)

  • In ancient India, opal was believed to be the Goddess of the Rainbow turned to stone

  • In Arab lore, opals are trapped lightning that fell from the sky

WHAT TYPES OF OPALS ARE THERE?

  • Two Categories: Precious Opal (has play of color) and Common Opal (no play of color) also known as “potch”)

  • There are over 100 varieties of opals that are classified by:

    • Color — black (dark tone), White, fire (bright orange or red), blue, cherry, yellow, pink, morado (purple), etc. (red is rarest)

    • Transparency — e.g. Crystal Opal is transparent, translucent, or semi-transparent, Water Opal (Hyalite) is a clear common opal

    • Mining Location — e.g. Australian (Andamooka, Cooper Pedy, Jundah, Koroit, Lightning Ridge, Mintabie, Quilpie, Yowah), Ethiopian, Honduras Black, Peruvian Blue

    • Struc ture

      • Boulder Opal — opal formed in seams within a host rock such as ironstone or andesite

      • Matrix Opal — opal evenly mixed within parent rock (e.g. Louisiana Opal)

    • Specific precious opal characteristics

      • Harlequin — has patches of color in the shape of rectangles or diamonds

      • Contra-Luz— color play visible when a light source is behind the stone

      • Pinfire or Pinpoint— has pinpoints of fire throughout the stone

    • Other characteristics

      • Chatoyance- Cat's Eye Opal has thin lines of fire

      • Plumes, Moss, or other inclusions — e.g. Opalite, common opal

      • Banding — e.g. Banded opal

    • Formation — Natural, synthetic, or Imitation (not silicon dioxide, usually plastic)

    • Trade names — e.g. Gilson Opal, Slocum Opal

  • Assembly method classifications

    • Solid — entire stone cut from a single piece of rough

    • Doublet— a thin layer of opal glued to a base of obsidian or basalt to increase strength and the play of color

    • Triplet— same as a doublet, but has a top layer of transparent material (such as quartz or spinel) for protection

WHERE IS IT FOUND?

95% of the world’s precious opal is mined in Australia (esp. S Australia, New S. Wales, Queensland)

Other countries that produce precious opal include Brazil, Czech Republic, Ethiopia, Honduras, Guatemala, Hungary, Indonesia, Mexico (known for fire opal), New Zealand, Peru, Poland, Slovakia, the U.S.  and Zambia

The most famous Iocations of precious opal in the US include Virgin Valley (Humboldt Co., NV), Spencer opal mines (Clark Co., ID), Opal Butte (Morrow Co., OR), and the Last Chance Opal Mine (Kern Co., CA)

The best states for precious opal are:  Arizona, Idaho, Louisiana, Nevada, and Oregon

Precious opal is very rare in Washington State. Previous mining districts include Whelan, WA (near Moscow, ID)

Common opal is found in abundance worldwide (including central Washington State)

WHAT IS IT USED FOR?

  • Jewelry — usually cabochon, but occasionally faceted

  • Diffcult to work due to crazing (the tendency to develop small cracks).  Requires special handling – storage in water with no exposure to extreme levels of light or temperature,  gradual drying process (sometimes for years)

  • Collector specimens

  • Diatomaceous opal is ground into a fine powder, known as “Tuller's earth" or “'tripoli", that is used for polishing

  • Common opal is used as an absorptive ingredient in pharmaceuticals and cosmetics and as an ingredient in bricks, sewer pipe, refractory mixes, fertilizers and insulation