Opal is unique
among gemstones. It inspires the strongest of emotions. Few gem
lovers are neutral about opal. One either loves opal or one fears
it. The Romans loved opal and considered it good luck. The Australian
aborigines, who associated it with the Rainbow Serpent, were fearful
of it. Marbodius, a medieval Bishop of Rheims, declared it to be
the patron stone of thieves and attributed to it the ability to
make the wearer invisible. The father of William the Conqueror attributed
all the bad luck of his house to the ownership of a single gem.
Today many people still believe that the wearing of opal is bad
luck unless it is one's birthstone.
The Roman historian Pliny identified India as the
earliest source of gem opal. There are early references to mines
at Poona, Bijapur and Sitibali. None of these sources has ever been
proved. The only documented ancient source of opal is the volcanic
deposit located southwest of the northern end of the Carpathian
Mountains in what is now Slovakia. These deposits were formed by
silica dissolved and deposited in cracks and veins of volcanic rhyolite,
trachyte and basalt.
There is very little evidence that gem opal was
known at all in the west prior to the Romans. No known written references
predate Pliny. There are no extant examples of opal jewelry from
European or Mediterranean cultures prior to Roman times. The word
itself comes from the Latin "opalus", the Greek "opallion"
is derived from the Latin. Modern discoveries of high-grade opal
in Ethiopia has led to speculation that this may be the Romans Asian
source. Opal from this source is, however, rather distinctive and
more of the Mexican type. The most likely scenario is that beginning
about 50 BC opal was traded from its Carpathian source down the
Danube river to the Greek city of Corinth and from there to Rome
Australia is the modern source of over 90% of the
world's opal. Australian opal was first discovered in Queensland
at Listoral Downs and at Springside in 1872. Commercial mining in
Queensland peaked in the 1890's, declining sharply after 1895 due
to drought. Deposits of opal were discovered at Whitecliff in 1890
and Lightning Ridge in 1902. The two major producing centers; Coober--
Pedy and Mintabe--were discovered in 1915 and 1919 respectively.
Coober Pedy was the single largest producer until it was surpassed
by Mintabe in the early 1990's. Coober Pedy is still the most important
producer of commercial grade white-based opal.
Photo: R. W. Wise
Open cut mining in the Australian outback. Here a bore is
made by a Caldwell Drill in an area outside Opalton, Queensland.
Mining is a hit or miss affair. If traces of color are found,
one of the miners will be lowered into the hole with a flashlight.
Unlike most gemstones, opal, like glass, is non-crystalline.
It is basically a silicate, 85-99% pure, with the chemical formula
Two important factors affect the stability of opal:
toughness and water content. Opal rates 5 1/2-6 on the Moh's scale,
although some stones from Brazil are close to 7. Opal is, therefore,
somewhat prone to scratching and abrasion. Toughness is another
matter; opal is rather brittle and subject to chipping. Gem Opal
contains between 6-18% water. Water content is important because
it directly relates to the stability of the gemstone. Opals with
high water content are unstable and more likely to craze or crack
There are six main types of opal classified according
to body color. Black opal has a dark gray to black body color. Additionally,
the stone must be opaque when held to the light to be classified
black opal. Semi-black has a dark body color but is translucent
when held to the light.
Crystal opal has a dark to light almost transparent
body color and looks like its name. White opal, the most common,
has a translucent to opaque milky white body color. Mexican opal
has a red to orangy-brown body color. Boulder opal may have any
body color but will include ironstone matrix as part of the cut
gemstone. Boulder opal has very low water content and may have some
legitimate claim to the title of most durable opal.
Color in gemstones is divided into three
components: hue intensity (saturation) and tone. Hue, what we usually
mean when we use the word "color” is the single most
important factor in defining value in most gemstones. For example,
the reddest red is the most important factor in the value equation
of ruby. In opal it is the brightness or intensity of the given
hue that is most important. In the words of one prominent dealer:
"opal is like a light bulb, the brighter it is the better it
Once we dispose of intensity, hue is the next most
important factor. There is a hierarchy of hues. It is generally
agreed that red is the top color, followed by orange and ending
with green. Here the agreement ends. Multicolor, an opal with three
or more hues is the most desired. Richard Drucker author of "The
Guide" a well respected gemstone price list, reserves his highest
rating for a stone that is seventy-five percent red and containing
two additional hues1.
Photo: R. Weber
By far the rarest and most expensive color, “red opal”,
a stone with fifty percent or more of a visually pure red
play of color will command the highest price. In addition,
these two stones show exceptional saturation.
ONE GLARING EXCEPTION:
Fire opal is graded differently from all
other types. In this type of opal body color is visually dominant
and is considered the key value factor. In fact, fire opal has little
if any play of color. Dennis Schmelzeneach of Opal International
suggests five most desirable qualities based on body color, ascending
from least to most expensive; these are: light orange, medium orange,
dark orange, orange/red and cherry or fire engine red. Other authorities,
notably Paul Downing, consider orange/red to be the top color.
Pattern is an important determinant of value. Again,
there is general agreement that harlequin, squarish blocks of color
which resemble the patchwork costumes of the clown figures featured
Picasso's early paintings is the most desirable. Pin-fire, a pattern
composed of myriad pinpoints of color, a sort of Technicolor Milky
Way, the least desirable. However, the question of pattern can be
made needlessly complex. It is rather a question of composition.
Pattern should be judged as if the gem were a fine abstract painting.
The play of color should be pleasingly distributed across the face
of the stone and appealing to the eye.
R. W. Wise
Traces of color in rough boulder opal.
This chunk of ironstone has been sliced on a saw.
THE CARE AND FEEDING
OF FINE OPAL:
Bring a few opal experts together or read
a couple of books and one thing is certain; disagreement. Some people
believe you should store opal in water others use glycerin. Both
practices are common in the opal fields. Others keep their opals
immersed in oil. While small amounts of oil rubbed into the surface
of a wear-pitted stone may temporarily enhance the clarity of the
stone, oil and water do not mix so oil immersion is not recommended.
Glycerin is actually a drying agent and although its immediate effect
may seem positive it may have a long-term negative effect on the
Opal is brittle and must be protected when setting.
Improper setting of opal in jewelry is a major reason why opal has
developed such a bad reputation for durability. Prong setting of
opal in rings is a common practice. It is also, unfortunately, a
prescription for disaster. The exposed edges of prong set opal are
much more likely to chip. Opal, especially fine stones, should always
be bezel set.