Jeweller - April 2022

BLACK & WHITE GEMSTONES | Colour Investigation
Obsidian
Obsidian is a form of natural glass and comes in numerous attractive
varieties. During prehistoric times, people have used obsidian to make
jewellery and carvings as well as cutting tools such as knives and spears.
Commonly found in large pieces, lapidaries frequently cut obsidian into
cabochons, beads, and carvings. The faceted pieces appear dark, except
when cut into small pieces and can be made into delicate jewellery stones.
This most popular variety is the snowflake obsidian which gets its name from
inclusions of snowflake-like spherulites of white cristobalite, frequently used
by jewellery-makers as beads and cabochons.
Pure obsidian is distinguished for its dark appearance, though the colour
varies depending on the impurities found in the stone. Iron and other
elements give the obsidian a dark brown to black colour, while some black
obsidians contain nano inclusions of magnetite- a form of iron oxide.
Only a very few obsidian specimens are colourless.
Black and white pearl
Considered as one of the best-loved gems of all time, pearls come in a
wide variety of colours, the most popular of which are white and cream
colour pearls.
Left to Right: Panda - Van Cleef & Arpels; Charms - Storrow
Natural pearls are formed when a microscopic irritant enters the bodies
of certain molluscs, while cultured pearls are done by inserting a bead
or piece of tissue that the mollusc coats with nacre.
Black, grey, and silver pearls are fairly common, with the main colour - or
bodycolour - is often modified to induce additional colours called overtones,
which are pink, purple, green, or blue.
Some pearls also show the iridescent light effect known as ‘orient’.
Pearls are commonly used for bead necklaces and bracelets or set in
solitaires, pairs, or clusters for earrings, rings, and pendants. Larger pearls
are also popular with creative jewellery designers.
Akoya cultured pearl farms are primarily found in Japan and China,
especially along the southern coasts of Guangdong and Guangxi provinces.
South Sea cultured pearls are farmed from the northern coast of Australia
through Indonesia to the southern coast of Southeast Asia, with large
operations in the Philippines as well. The Gambier Islands and the Tuamotu
Archipelago – both in French Polynesia - are two of the major sources of
rich black Tahitian pearls.
Chalcedony
Chalcedony is named after the ancient Greek seaport ‘Chalcedon’
(khalkedon) in Asia Minor and the Latin word ‘chalcedonius’.
Chalcedony, which is found worldwide, is a microcrystalline variety
of quartz, meaning its microscopic quartz crystals require high
magnification for it to be seen.
When crafted as jewellery, chalcedony is distinguished by its light blue
translucent and waxy form, which can be found in be many colours -
chrysoprase is a popular example, the sought-after apple green variety.
Ideal for carving cameos, intaglios, ring bezels and beads, Chalcedony
has been used since ancient times in the Mediterranean region and
Central Asia.
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03 9654 4449
Level 4, Wales Corner
227 Collins Street
Melbourne VIC 3000
gems@kandk.net.au