Black Opal  OPAL

   From the Latin opalus, meaning precious stone. Opal is the birthstone for October (also tourmaline). Unique among all gems because of its source of color (diffraction). Opal is amorphous (i.e. noncrystalline), hydrated silicon dioxide (SiO2 * nH2O), that contains about 3-15% H2O by weight. The source of the play-of-color in opal is the semiperiodic arrangement of spheres of hydrated SiO2, whose centers are spaced at about the wavelength of visible light. These serve as a diffraction source for refracted and reflected light. Finer quality opal (that exhibiting great play-of-color) generally has more H2O than lesser quality material. Water content can be quite variable however, even within a single piece. The relatively high water content of most opal leaves it susceptible to damage by rapid or extreme changes in temperature. Opal mined from a damp or wet source, or that mined from volcanic host rock is more prone to dehydration than that mined from a dry source or from a sedimentary host rock. Dehydration may result in cracking (referred to crazing or checking) that greatly diminishes its value.


Properties

  • Crystal System: None; amorphous
  • Habits: Anhedral (i.e. no crystal faces); as thin crusts lining fractures in siliceous rock, as nodules in a volcanic matrix, as vein material, or as crust replacing silicified fossils.
  • Hardness: Depends on water content (d.o.w.c.); usually 5.5-6.5.
  • Specific Gravity: D.o.w.c.; usually around 2.15 but ranging from 1.98 to 2.20.
  • Toughness: Poor to good, d.o.w.c.
  • Cleavage: None
  • R.I.: D.o.w.c.; 1.42-1.47 (1.44-1.46).
  • Isotropic
  • Color: Body colors black, gray, white, orange, red (cherry, fire), yellow, transparent colorless (hyolite, jelly opal).

Distinguishing Properties

    Nothing else shows the play-of-color of precious opal. Transparent Opal can be distinguished by the extremely low S.G. and R.I.. Synthetic Opals manufactured by the Gilson Co. are outstanding for their bright, large patches of color. These features, which are uncommon in all but the best natural opal, as well as highly irregular color boundaries (snakeskin- or honeycomb-textured at 50-60X magnification) and a somewhat ordered appearance to the color patches, are usually sufficient to tell a synthetic from a natural.

Varieties

A wide variety of names have been and continue to be used to describe various types of opal. Most have their basis in three important attributes; 1) body color; 2) transparency; 3) character of play-of-color and colors present. The following names are in common use:

  • Black Opal- black, dark green, dark brown or other dark body color with vivid play-of-color. Most valuable. Often synonymous with " Australian Opal" or "Coal Black".
  • Crystal Black Opal - Black opal that is transparent to translucent.
  • Crystal Opal - transparent to translucent, having no body color but strong play of color.
  • White Opal - translucent to opaque, with white body color and play-of-color. Once referred to as "Hungarian Opal".
  • Boulder Opal - opal with play-of-color that is present in a matrix of dark brown or black sandstone (ironstone).
  • Transparent or Semitransparent Opal - transparent opal with slight to no play-of-color with a body color that is yellow, orange, brown, red or colorless. Colorless called "Water Opal", "Jelly Opal", "Hyalite", "Contra-luz", "Hydrophane". Yellow, orange and red body color referred to sometimes as "Fire Opal" or "Cherry Opal". This material is commonly faceted rather than cabachoned.
  • Common Opal ("Potch") - translucent to opaque, having nearly any body color, but no play-of-color. Known by a myriad of names; e.g. Matrix Opal, "Geyserite", "Wood Opal", etc.. "Common Opal" contrasts with "Precious Opal" in the categories above.

 Secondary Classification - based on how play-of-color looks:

  • Pinfire or Pinpoint - very small, closely spaced, color flashes.
  • Mosaic or Harlequin - large (ca. 2 mm or greater), regular, angular, patches of color.
  • Flame - sweeping reddish bands, like a wind-blown flame.
  • Flash - sudden play-of-color as stone moved.
  • - several other terms as well

Sources and Origin

Formed at low temperatures (<200oC) by the precipitation of colloidal silica from groundwater moving through siliceous rocks in arid regions, where the water table can experiences great seasonal and long-term fluctuations.

  • Alteration of siliceous sedimentary rock (quartz sandstones) along cracks and cavities to deposit opal crusts or nodules - Australia
  • Alteration of siliceous volcanic rocks - Mexico
  • Also by the concentration of silica gel derived from marine organism in evaporating sea water; replacement of skeletal remains and shells by opal.

Primary Producers

  • Australia - worlds most important source (discovered 1872).
    • Lightning Ridge District, New South Wales - since 1905; exceptional Black Opal.
    • Coober Pedy, South Australia - Aboriginal name meaning "white man in a hole" in reference to miners living accommodations at the site, where they lived in dugouts or bunkers to escape the heat. White Opals only, since 1915. Worlds largest and richest producing area.
    • Andamooka Station - since 1930, White Opal. Not much production today.
    • Mintabie - since 1921; significant producer only since 1976 (remote, inhospitable area). Black Opal (gray to grayish black body color) and white opal.
    • Several lesser areas in Queensland - noted for "Boulder Opal" (see above) and "Yowah nuts"; hollow, walnut-sized ironstone concretions.
    • Lambina - minor production from 1957 to 1977.
    • - Coober Pedy and Mintabie account for over 80% of the worlds precious opal production.
  • Mexico - source of much Fire and Cherry Opal and some fine White Opal. Areas near Mexico City produce from rhyolitic volcanic rock.
  • Czechoslovakia - "Hungarian Opals" - part of Hungary prior to W.W.II. One of the earliest source, now worked out. White Opal only.
  • U.S. - most important area is in northern Nevada (Virgin Valley); opal here replaces petrified wood in volcanic ash. Has extremely high water content, get beautiful black opal with red flashes. Is notorious for dehydrating with time, however, causing cracking, crazing and making it less marketable than comparable material from Australia.
  • - Also areas in Oregon (Hyalite), Honduras, Texas (near Alpine), Idaho.

Shaping and Polishing

Translucent to opaque material cut en cabochon; transparent is faceted. Extra care needed in cutting and polishing due to heat sensitivity. Boulder Opal cabochons often retain much matrix because of the irregular and sparse distribution of the opal within.

  • Cabochons
    • Standard
    • Doublet
      • Used for very thin material. Opal crust cemented to backing (glass, chalcedony, rough opal, sandstone matrix, etc.), often with black glue to give appearance of black opal. Seam often hidden by mounting.
    • Triplet
      • Same but with a protective, clear cover (often quartz or synthetic spinel) over the opal layer.
  • Faceted
    • Cherry opal, hyalite, some fire opal. Check for play-of-color.

Pricing and Valuation

Sold by stone if doublet or triplet; by weight if single piece. Beware of doublets if paying by weight.

  • Factors influencing price
    • Intensity and distribution of play-of-color.
    • Body color
    • Number of colors present and relative abundance
    • Size, shape, weight of cab..
    • Amount of matrix vs. opal in cab.
    • "Ideal" is black opal with individual, vivid, angular patches of color of varying hues, with patches large enough to be seen at arms length.
  • Colors in order of value (Australian Opal):
    • Red
    • Violet
    • Orange
    • Yellow
    • Green
    • Blue
    • For Mexican Opal: Red, Violet, Green, Blue

    Harlequin black opal that shows lots of red is most highly prized; increasing amounts of blue and green reduce its value.  Lacking this, pinfire opal is the least expensive.

Prices

    Because Opal grading involves so many different factors, prices show an extremely large range for material that is given the same name. Black Opal, for example, may sell for as little as $40/carat or as much as $5,000/carat. Below are some very general price guidelines (wholesale, 1990):

  • Black Opals - absolute finest material about $3000/carat in 5-6 carat sizes. Smaller stones $1-2K/carat less, larger ones more. Commercial material (blue and green flashes only, some potch, poorer pattern definition) about $50-$250/carat.
  • "Crystal" Opal - $15 -$1000ct in 1-15 ct sizes
  • White Opal - $1-$200/carat, depending on size and factors discussed above.
  • Boulder Opal - difficult to price by weight because of variable amounts of matrix. Should maybe be priced by the stone instead. General guides to stones with less than 50% matrix are perhaps $500-$1000/carat for stones of about 5 carats, with smaller ones about 75% of this and larger ones 1.5-2X these prices.
  • Doublets and triplets - $3-$200/stone.
  • Fire or Cherry Opal, faceted - $30-70/carat.
  • Gilson synthetic opal (1989, wholesale) - $40/carat.

    One source states prices for fine black and white opal have risen about 20% per year for the last 20 years.


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1998
Updated 08/20/09
Comments and questions to helper@mail.utexas.edu
Department of Geological Sciences
The University of Texas at Austin