Jeweller - June 2021

Colour Investigation | MULTI-COLOUR & COLOUR CHANGE
GEMSTONE FOCUS
THE OPULENT OPAL
from reddish-purple to a steely blue, green or greyish tone. Other
possible colour combinations are reddish-orange to red, greenishyellow
to pinkish-red, light-red to purplish-red and bluish-green to
light violet-purple.
Diaspore
NORTHERN
TERRITORY
Winton
QUEENSLAND
Rounding out the top colour-change gemstones is colour-change
diaspore; called “a true Turkish delight” by the International
Colored Gemstone Association (ICA), gem-quality specimens were
discovered in the Turkish Anatolian Mountains in the 1970s at
heights of more than 1,200m (4,000 feet).
WESTERN AUSTRALIA
Coober Pedy
Yowah
Natural and untreated, high-quality colour-change diaspore
has been sold under various trade names, all referencing the
Sultans, Tsars and Ottomans of Turkey – these names include
zultanite, csarite and ottomanite.
Irene Neuwirth
SOUTH AUSTRALIA
NEW SOUTH
WALES
VICTORIA
Lightning Ridge
A well-kept secret is Australian colour
change sapphires from the central
Queensland gemstone fields that
display some unusual colour changes
– brown to green, yellow to pink, or
golden orange to orangey-green
PASSION
COLOUR
EXPERIENCE
Tiffany & Co.; John Dyer
Suite 5, Level 1, 428 George Street SYDNEY NSW 2000
P +61 2 8065 8533 E info@sovereigngems.com
@sovereigngems
Precious opal is composed of minute uniform spheres of silica,
which are arranged together in an orderly three-dimensional grid.
The spaces between these spheres contain silica in solution. White
light passes through the transparent spheres directly, but when it
TAS
reaches the silica in solution, it is deflected at angles.
These diffracted beams of light may show all the colours of the
spectrum, or particular colours may predominate. The colour from
the opal is dependent on the size of the spheres, which determine
the wavelengths.
For instance, blue colours are evident where the spheres are
smaller and, at the other end of the spectrum, orange and red will
be evident where the spheres are larger. The intensity and brilliance
of the colours are a result of the degree of uniformity of size and
regularity of the grid.
The value of an opal is determined by the type of opal, the
predominant colours it exhibits, the clarity or brilliance
of these colours, and the patterns in which the colours
are arrayed. Good patterns of diffracted colours have an
enormous impact on the value of the opal. Pinfire and small
type patterns are more common, and thus less expensive
than broad patterns.
The major outcrops of opal in Australia occur along the shoreline
of what was once The Great Inland Sea. The Queensland fields are
spread across 1,000km and produce almost all the world‘s supply of
boulder opal.
Lightning Ridge produces almost all of Australia’s sensational black
opals and, despite a recent decline in production, it is still the largest
producer of opal by value.
The bulk of the world’s light seam opal has been mined at Coober
Pedy. By 2008 Coober Pedy’s production had fallen to 15 per cent of
its heyday but this dusty, hot outback town remains home to more
than 4,000 people.
Source: Cody Opal
George
Pragnell
Diaspore’s first patented trade name, zultanite, was registered in
2005 by Turkish jeweller Murat Akgun in honour of the 36 sultans
who ruled the Ottoman empire from 1299 to 1923.
Regal associations are certainly befitting this exotic, pastel-toned
gemstone coloured by manganese. Traces of chromium in the
presence of iron causes colour changes depending on the light.
What appears as kiwi green with flashes of yellow in sunlight
might seem raspberry or brownish pink under candlelight,
champagne in incandescent light, and something else entirely
in other light sources.
Another feather in diaspore’s cap is a property known as
trichroism. These colours – brownish pink, yellowish green
and sometimes violet blue – are distinct and contribute towards
its colour-change effect.
A factor impacting the value and availability of colour-change
diaspore is its tendency to cleave, which presents a challenge for
the cutter. Yield rates are notoriously low and can be as little
as 2 per cent for eye-clean material and 10 per cent for larger
sizes; consequently, large, clean and well-cut gemstones are
extremely rare and expensive.
Opal and ammolite
Like colour-change diaspore, ammolite is also exclusively found in
one location – a region in the Rocky Mountains of North America.
Ammolite forms when the fossilised shell of ammonites – nowextinct
marine invertebrate animals – are preserved and the cavities
that originally held the soft body are filled with aragonite, the same
mineral that is responsible for the nacre of pearl oyster shells.
The ammonites that form ammolite specifically inhabited a
prehistoric inland subtropical sea that bordered the Rocky
Mountains; as the sea receded, layers of sediment preserved
the shells.
Lydia Courteille
Leviev
Van Cleef & Arpels
Featuring the
delicate pink tone of
Argyle Featuring pink diamonds the
delicate pink tone of
Argyle pink diamonds
SAMS GROUP
AUSTRALIA
E pink@samsgroup.com.au
W samsgroup.com.au
PE
02 pink@samsgroup.com.au
9290 2199
W samsgroup.com.au