Jeweller - April 2022

Colour Investigation | BLACK & WHITE GEMSTONES
Left to Right: Christie Nicolaides; Verdura; Mateo NY; Chopard; Moon Tree Baby; Verdura
Onyx
For centuries, artisans have carved intricate and
attractive cameos using black and white onyxthe
most popular of which are cabbed or faceted
solid black onyxes.
A type of chalcedony, onyx is a form of
microcrystalline quartz that is distinguished by
its straight, nearly parallel layers or bands of
colour, allowing highly-skilled gem carvers to
work on layers to create intaglios and cameos
with extraordinary depth and contrast.
The most common onyxes are those with white
and black layers. The gems with a black colour
base and a white upper layer are called “true” or
“Arabic onyx.”
The onyx, especially the black variety, was regarded
as a popular gemstone during the Victorian era
(1861-1885), particularly as “mourning” jewellery
and although it still bears the same reputation
in some Western cultures, its dark and opaque
appearance is used to complement clear and
coloured gems – including diamonds - in a variety
of jewellery pieces.
Moonstone
Moonstone is a variety of feldspar composed of two
minerals, orthoclase and albite. The orthoclase
and albite separate into alternating layers during
formation, such that when light falls between these
thin layers the rays get scattered to produce a
phenomenon called adularescence.
Adularescence is the sheen that radiates from
beneath a gem’s surface and appears to move
along with the light source, producing an effect
resembling moonlight on water.
The delicate beauty of the moonstone makes it
one of the most popular gem-quality varieties of
feldspar minerals.
The clarity and colourless appearance of the
moonstone and the blue light intensity of its
adularescence gives the stone a higher value.
A June birthstone, moonstone is revered for
its white and bluish light effect and while it is
somewhat fragile, it is a popular choice as a
centre stone.
Labradorite
Labradorite is a calcium-enriched feldspar mineral
discovered in Labrador, Canada which possesses an
iridescent optical effect known as labradorescence.
The mineral is made of thin and tabular shapes
with colours ranging from purple to grey-blue,
green, yellow, and brown, which has a pearly
lustre and some specimens produce a distinctive
schiller effect.
White labradorite – also commercially known
as rainbow moonstone – is a popular jewellery
piece due to light effects that reflect on a white
background.
Some labradorite varieties are known to
exhibit high degrees of labradorescence called
spectrolite and are among the most preferred
types used for jewellery.
White and black opal
Ancient Romans considered opals as the most
valuable among precious stones since it was
known to have many colours that were also
found in other gemstones.
Opals are known for their unique play-of-colour,
which determines how precious the stone is.
The white or light opal is distinguished as
translucent to semitranslucent, with play-of-colour
against a white or light grey background colour.
While it is considered to be the least expensive
among other precious opal varieties since it is the
most common, it can display any colour of the
spectrum to reflect a beautiful play-of-colour.
On the other hand, the black opal is considered the
most valuable variety of opals due to the stone’s
dark body tone and vibrant play-of-colour.
Distinguished as translucent to opaque, with playof-colour
against a black or other dark background.
Black opals are hard to find and can fetch prices of
up to $AU15,000 per carat.
In the 1980s, Australia became the world’s major
source of opals when mines began commercially
producing the stones.
54 | April 2022