Valued for its shifting
play of colour, opal’s rainbow hues offer the colours of all
the other gems combined. Opal’s play of colour is created by
internal diffractions: tiny silica spheres in grids scatter
light into shimmering iridescent spectral colors, much like
the tiny grooves on CDs.
History and Lore
The name opal is a
shortened version of opalus, a Roman coinage that sums up
this gem’s chief aesthetic attribute : colour play.
Opal was treasured in
the Middle Ages and was called opthalmios, or eye stone, due
to a widespread belief that it was beneficial to eyesight.
Some thought the opal’s effect on sight could render the
wearer invisible.
Opal was said to
protect blonde hair from losing colour.
Opals are featured in
the crown jewels of France. Napolean gave Josephine a
beautiful opal with brilliant red flashes called “The
Burning of Troy”.
In the nineteenth
century, opal was considered unlucky due to the plot of the
popular Sir Walter Scott novel of the time. The heroine of
the nove; jas her life force caught in the beautiful opal
she wears in her hair and she dies when the fire in the opal
is extinguished.
Opal is one of the US
birthstones for October.
Quality, Value and availability
When assessing opal,
experts start with its background body colour. A darker
overall base colour with vivid play of colour is considered
the most desirable.
The most valuable opal
has a dark gray to black body colour: this is known as black
opal.
Opal with a white or
light body colour, usually called white opal, is much more
common and affordable.
Opal with a very
translucent to transparent body colour is called crystal
opal and is valued in between white and black opal.
After body colour,
opla’s value is based on the amount and distribution of play
of colour. The ideal is broad patterns covering the surface
of the stone, with all the colours of the spectrum,
including red, represented. But opals are the most
individual of gems and personal taste should also guide your
selection.
Sources
The most important
source of opal is Australia, which produces the most opal
and finest opal. The legendary locality for the best black
opal is Lightning Ridge.
Fire opal, a bright
yellow to red transparent volcanic opal mined primarily in
Mexico, is usually faceted. Fire opal occasionally can also
display play of colour but it is more rare. Fire opal is
also found in Oregon in the United States.
Opal is usually cut in
smooth-domed oval cabochon shape. Although white opal is
found in standard sizes, black opal is of valuable that it
is cut in whatever shape will maximize its play of colour,
although ovals are preferred.
Routine Enhancements
The most common opal
enhancements are methods of darkening light opal to make it
resemble black opal. In dyeing, a coloured substance is
introduced into the porous structure to produce a more
desirable colour appearance. “Smoke” treatment produces a
dark brown to black body colour. “Sugar” treatment produces
an opaque black body colour.
Pieces of opal that are
too thin to cut into gems for jewelry are sometimes used to
create a sandwich-layered assembled stone called a doublet
or triplet. This assembled stone has a very thin layer of
opal with a stone backing and sometimes also a colourless
quartz or plastic dome. Doublets and triplets are valued
less than solid opal. They are sold per piece and can be
priced in the hundreds for a spectacular example.
Care and Cleaning
With a hardness of 6 on
the Mohs scale, opal is softer than many other gems and
should be stored carefully to avoid being scratched by other
jewelry.
This gem should also be
protected from blows, as exposed corners can chip.
Opal should not be
exposed to heat or acid.
Clean opal with mild
dish soap in warm water : use tooth brush to scrub behind
the stone where dust can collect.
