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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