Jeweller - August Issue 2018

GEMS
COLOUR INVESTIGATION: TOURMALINE – PART I
crystals, allowing some unusual long slender
cuts. Pink to red crystals often have more
visible inclusions than other coloured varieties
although clean crystals are not uncommon in
sizes under 2 carats.
IMAGE COURTESY: GIA.EDU
Boasting an array of colour and colour
combinations unrivalled by any gem
species, one can find tourmaline in shades
of almost every hue. Throughout history
this gem was often confused with other
minerals until tourmaline was identified
as its own mineral species in the 1800s.
STACEY LIM reports.
The name ‘toramalli’ (the Sinhalese word for
“gems of mixed colours”) was originally used
to describe these unknown coloured crystals.
The tourmalines are a group of mineral
species called borosilicates, that is, they are
silicates containing boron, but also a mix of
other elements.
Consequently, they have a similar crystal
structure but their varying chemical
composition means they have different
physical properties. In particular, the inclusion
of other elements, such as chromium,
iron and manganese, creates the variety of
colour for which tourmaline is famous.
The major tourmaline species are elbaite,
liddicoatite, dravite, uvite and schorl, but most
gem tourmalines are elbaites, which contain
sodium, lithium and aluminium. Coloured by
traces of various other elements, elbaites can
be blue, green, yellow, pink to red, colourless
or zoned with a combination of colours.
Traces of manganese, for example, are
thought to produce reds and pinks,
although pinks may also owe their hues to
colour centres caused by either natural or
laboratory-induced radiation.
Some varieties of tourmaline flaunt trade
names such as ‘rubellite’, the name given to a
range of red tourmalines, often tinged with
orange, purple or brown. Some members of
the trade argue that terms such as rubellite
are archaic and varieties should be named
after their colour and species alone.
Tourmaline typically forms as long prismatic
NEARLY ALL
TOURMALINES
DISPLAY
DIFFERENTIAL
ABSORPTION OF
LIGHT, WHERE
THE HUE AND
SOMETIMES TONE
VARY WITH THE
ORIENTATION OF
THE CRYSTAL
Nearly all tourmalines display differential
absorption of light, where the hue and
sometimes tone vary with the orientation of
the crystal. Pink crystals appear darker pink
in the direction of the optic axis (parallel to
the length of the crystal), and lighter pink
perpendicular to the optic axis. This enables
a degree of control over the depth of colour
when fashioning a stone. Carefully controlled
heating or irradiation can also alter colour,
and while the results of both methods are
rather uncertain, these treatments are still
extensively used commercially, and are often
very difficult to detect.
Known for showing multiple colours in
a single crystal, ‘watermelon tourmaline’
is a particularly desirable parti-coloured
variety. Looking exactly as it is named,
these impressive crystals show pink in the
centre, running down the crystal axis with a
concentration of green around the outside.
It is typically cut into slices to display this
astonishing colour arrangement.
The warm hues of tourmaline display a
wide range of colour intensity and tone and
while rubellite is generally a more available
coloured variety of tourmaline, those with a
hue and depth of colour that approach the
intensity of fine ruby and pink sapphire can
attract higher prices.
A vibrant and impressive gemstone,
tourmaline will continue to rival its precious
counterparts with a seemingly endless array
of colours. i
STACEY LIM FGAA BA Design, is a qualified
gemmologist and gemmology teacher/assistant.
She is a jewellery designer, marketing manager
and passionate communicator on gemmology.
For information on gemstones, visit: gem.org.au
August 2018 Jeweller 49