Jeweller - October 2019

GEMS
GARNET – GEM OF MANY COLOURS: PART I
greens. There is also a colour change garnet,
which shows blue-green in daylight, shifting
to purple-red under incandescent light.
Why does garnet have so many colours?
The answer relates to chemical processes as
the garnet crystal is forming. Simply put –
because the chemistry of the garnet family is
complex – in some garnets, oxides of metals
such as iron, chromium and magnesium are
inherent to the crystal, thus creating specific
colours.
In other garnets, trace elements are included
as the gem form, thus creating a different
range of colours.
FIGURE 1. CORROSION TUBES
FIGURE 2. LAMELLAR TWINNING
The garnet family is an extensive one,
with 20 members. In the jewellery world,
five members are of importance, namely:
pyrope, almandine, spessartine, grossular and
andradite. Each of these will be considered
next month in Part II of the Garnet – Gem Of
Many Colours series.
Long an indicator of wealth and status,
deep red garnet was coveted by monarchs
and nobles across many ancient cultures.
The Romans used carved garnets in seals
to mark their official documents, the
ancient Britons decorated their weapons
with the gems, and Egyptian pharaohs
were buried with strings of garnets.
The name garnet comes from the Latin word
granatus or granum meaning “seed”, as red
garnet in its rough state is similar in colour
and shape to the seed of the pomegranate.
Garnet has a long history of use as a
decorative as well as a practical item. Its
hardness of 6.5 to 7.5 on the Mohs scale,
durability and vibrant colour palette make it
ideal for use in jewellery as well as a range of
ceremonial regalia such as crowns, chalices
and sceptres.
Hardness and durability, key properties
valued by jewellers and artisans for
millennia, also make garnet an ideal modern
industrial resource. Originating in volcanic
and metamorphic environments and thus
subjected to extreme heat and pressure,
garnet can withstand similar extremes in an
industrial setting.
Today, industrial grade garnet is widely
used as an abrasive, in high-pressure water
jet cutting tools, as a component of wear
resistant road paints and in rechargeable
batteries. Australia is a leading supplier
of industrial grade garnet, with mines in
Western Australia and the Northern Territory.
Gem quality garnet is typically viewed as
a red gemstone, with colour variations of
brownish red to reddish pink. However, this is
only one part of the gem’s colour story.
Many customers are surprised to learn that
this affordable gemstone comes in a rainbow
of hues, including colourless, blue, black,
orange and yellow, purple and a range of
WHY DOES
GARNET HAVE
SO MANY
COLOURS? THE
ANSWER RELATES
TO CHEMICAL
PROCESSES AS THE
GARNET CRYSTAL
IS FORMING.
SIMPLY PUT –
BECAUSE THE
CHEMISTRY OF THE
GARNET FAMILY
IS COMPLEX
Colour aside, garnet can also display the rare
optical effects of chatoyancy (cat’s-eye effect)
and asterism (star effect). Such garnets are
highly sought after. Some garnets may also
contain iron, giving these gems magnetic
properties. In addition to its colour range,
garnet has a bright vitreous lustre.
One garnet type, the vibrant green
demantoid, has a dispersion value greater
than diamond, adding to this garnet’s
gemmological and jewellery value.
Gem quality garnets are found across the
world, including in Brazil, Australia, Myanmar,
Madagascar, India, Sri Lanka and Namibia.
SUSAN HARTWIG FGAA came late to the world
of gemmology after a long career in corporate
training and project management. She
combines her love for writing with a passion
for gems and jewellery. Susan writes regularly
for her gemmology blog ellysiagems.com. For
more information on gemmology courses and
gemstones, visit: gem.org.au
October 2019 Jeweller 37