Jeweller - July 2022

AUSTRALIAN GEMSTONES | Colour Investigation
Left to Right: Raw seam opal stack
from White Cliff opal fields; White
Cliff opal pineapple discovered by
Grant Pearson; Paspaley
weathering of serpentine and consists mostly of
silicon dioxide.
As the elements break down, the serpentine,
nickel, silica and other iron oxides are dissolved
out of stone and collected in crevices, cracks and
clay. Although most green stones owe their colour
to chromium or vanadium, chrysoprase derives
its colour from the presence of disseminated
particles of hydrated nickel silicate and ranges
in colour from apple green to a deep rich green.
It is usually translucent but unlike many nontransparent
members of the quartz family the
colour of chrysoprase, rather than their patterns
or markings, makes it more desirable and
valuable.
Due to its bright, even colour and texture,
chrysoprase is often cut into cabochons, beads
and a base for intricate carvings. It is popularly
used for intaglios and cameos, dating back
to Greek and Roman times and used lavishly
throughout Europe until the middle of the last
century, when it became rare and more expensive.
Chrysoprase is primarily sourced from central
Queensland in Australia and due to its abundance,
is often referred to as “Australian jade”.
The best quality chrysoprase is produced from
the Marlborough deposits near Rockhampton
in central Queensland. The gemstones are also
found in numerous locations in Western Australia
such as the Marshall Pool, Yerilla, Yundamindra,
Jamieson and Wingellina deposits.
The name ‘chrysoprase’ is derived from the Greek
word “chryos”, meaning golden and “prason”,
meaning leek or green. The gemstone was given
its name in the 18th century when a vein was
discovered in Silesia – which is now Poland - at
the time of Frederick the Great of Prussia, who
held a particular fondness for the gem.
Among his favourites was a walking stick adorned
with a knob of chrysoprase and a ring set with a
large chrysoprase surrounded by 15 diamonds.
The gemstone was believed to bring supernatural
and spiritual protection, prevent depression,
promote equilibrium and increase fertility.
Due to its excellent durability, chrysoprase is
widely used in jewellery-making. It is rated 6.5 to
7 on the Mohs hardness scale, making it resilient
and suited for jewellery and carving.
It is easily handled for creative and artistic
designs as it takes on a fine polish. The toughness
of the gemstone makes it a very forgiving stone
and allows for carvings and fine workmanship
that brings out the brilliance when reflecting
ambient light.
Chrysoprase is primarily sourced from
central Queensland in Australia and due
to its abundance, is often referred to as
“Australian jade”.
Chrysoprase jewellery can be worn daily and
maintained using an ultrasonic jewellery cleaner
or with warm, soapy water and a soft brush. The
gemstone’s vibrant hue can fade in the sunlight
over time due to dehydration, which deprives the
gemstone of its colouring agent. The gemstone
can lose its lustre when it dries but can be
restored by soaking in water.
Zircon
Zircon is considered to be the oldest mineral on
earth and Australia has the largest and oldest
deposits, dating back more than 4.4 billion years.
The most prominent source of Australian zircon
is the Mud Tank Zircon Field in the Harts Range
area situated some 1,220 kilometres south to
southeast of Darwin in the Northern Territory.
First explored in the 1940s, the area is well-known
amongst fossickers and speculators for
its top-quality zircon, specifically from two
locations the Zircon Hill and Specimen Hill.
Waterworn zircon crystals occur mostly in the
low-lying areas between the two hills.
Prized for its diamond-like lustre, intense fire,
brilliance and strong double refraction, zircon
sets itself apart from the rest of the gem-quality
minerals.
The zircons of Australia display beautiful earthy
tones of sherry, cinnamon, cognac, pinks, plums,
yellow, and orange, as well as gemstones that are
partially coloured and colourless.
Zircon may occur as small, doubly terminated
crystals or chips, and larger specimens may show
well-developed crystal faces and are often found
intermixed with sapphire in deposits associated
with tertiary volcanic deposits in Tasmania and
northern Queensland.
The zircon gemfields of northern New South
Wales and central Queensland produce
gemstones ranging from colourless to
champagne and orange-red.
Australian zircons are often heat-treated to
lighten, or to make them colourless, and are
prized for their lustre, brilliance, colour intensity,
and strong double refraction. Golden brown
gemstones can be similar to citrine or topaz.
Zircon contains traces of uranium, which
allows for self-irradiating and the changing of
properties. It is classified into three types – high,
intermediate and low. The gemstones are rated
7 to 7.5 on the Mohs hardness scale.
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