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Gemstone Special Effects (SFX)
"To suppose that the eye... could have been
formed by natural selection, seems, I freely
confess,
absurd in the highest degree".
Charles Darwin (1809-1882), Origin of Species
Gemstones, jewels, bijous, rocks, and 'bling'
are much more than just dazzling, multicoloured
expensive minerals and organics. Some gemstones
also have special optical effects or simply 'SFX'.
Called 'phenomena' by gemmologists, these rare
and beautiful optical properties are part of a
gemstone's structure. Frequently adding value,
expert gem cutting can bring out these unique
effects to their fullest, or hide them when
necessary. So grab the popcorn and turn off your
mobile, 'The Clever Gem Buyer' cinema is about
to present, 'Gemstone Special Effects'...
Adularescence (Opalescence)
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Adularescence mightn't be the
easiest word to remember, but it is very
easy to spot. Remember 'Moon Shadow' by
Cat Stevens? Adularescence is the silver
to bluish-white light that shimmers and
glides over the surface of that
enchantingly romantic gem, Moonstone.
Adularescence is due to 'interference
phenomena', which simply means the
scattering of light by thin crystalline
layers. This effect gets its name from a
variety of Moonstone found in the
European Alps called 'Adularia', and is
called 'opalescence' when occurring in
Opals. |
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Rainbow Moonstone showing its
characteristic blue adularescence
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Asterism
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Imagine
being the first person to look into a gem and
see a luminous star. No wonder star gems were
once regarded with so much superstition.
Asterism, also known as the star effect or
asteria, is a uniquely beautiful, mysterious
optical effect especially coveted in Rubies and
Sapphires. Asterism is created by the reflection
of light from multidirectional, long
needle-shaped inclusions that occur in parallel
arrangements in at least two different
directions. While they come in six rays, four
rays, and on the odd occasion 12 rays, all star
gems are dependent on a gem being cut 'en
cabochon' (cut in convex form and highly
polished, but not faceted). Asterism is most
visible in a direct, single beam of light.
Colour aside, a well cut star gemstone has a
distinct star whose rays are straight and
equidistant. Stars are typically positioned in
the centre of the gem, bearing in mind that they
are sometimes deliberately off-centred for
aesthetics. |
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Star Ruby, once aptly known as the
'three swords' |
Aventurescence
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Another
'scence' word, this one originated in an
18th-century Venetian glassworks, when copper
filings accidentally fell into a batch of molten
glass. According to legend, the workers
exclaimed, "a ventura", which means 'by chance',
on noticing their glass with sparkles. In
gemstones this phenomenon is caused by
glittering metallic inclusions. In Chalcedony
Quartz, thousands of tiny metallic flakes create
the glimmering rarity called Aventurine and when
present in Feldspar, sparkling Sunstone. One
'gem' to be aware of is Goldstone, a manmade
glass and copper substitute. |
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Madagascan Sunstone |
Chatoyancy
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French
for cat, le chat is obviously all about our
feline friend. Chatoyancy, also known as the
cat's eye effect, appears as a single bright
reflective line of light, similar to a cat's
eye. Chatoyancy is created by the reflection of
light from long needle-shaped inclusions
occurring in a parallel arrangement. Similar to
star gems, the cat's eye effect is dependent on
a gem being cut 'en cabochon' and is most
visible in a direct, single beam of light. One
gem synonymous with this phenomenon is
Chrysoberyl, so much so that that if you just
say 'cat's eye' to a gem professional, they'll
assume you mean Cat's Eye Chrysoberyl. |
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Cat's Eye Chrysoberyl |
Colour Change
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Manfred
Eickhorst M. Sc. Physics, well-known for his
innovative lighting and gemmological equipment,
states, "The beauty of gems to the human eye is
determined by the gemstone's colour appearance".
Obviously, this beauty is dramatically enhanced
if a gem can change its colour. Gemstones that
display this phenomenon include the miraculous
Alexandrite, which can appear blue-green, forest
green, green, khaki, teal or yellowish-green,
and combinations thereof in candescent light
(sunlight), and brownish-red, orangey-red, red
or reddish-purple, and combinations thereof, in
incandescent light (candlelight). Also know as
the 'Alexandrite Effect', colour change gems
show different colours when viewed in two
different light sources. As revealed by
rainbows, white light is comprised of the
individual colours of the spectrum: blue, green,
orange, red, violet, and yellow. When white
light passes through a gem, some of these
spectral colours are absorbed. The spectral
colours of white light least absorbed combine to
produce the colour of the gem. It is changes in
a light source's component colours that cause
gems to look different in various lighting
conditions. Although they appear similar to our
eyes, sunlight has very strong blue wavelengths,
while electric light is richer in red
wavelengths. In colour change gems, absorption
of different colours of the spectrum from
different light sources results in our 'colour
change' perception. Other colour change gems
include Colour Change Garnet, Colour Change
Sapphire and Zultanite. A colour change gem is
rated by the strength of its change, and how
attractive and distinct its colour is in both
candescent and incandescent light. |
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Brazilian
Alexandrite showing its
colour change |
Iridescence
Derived
from the Greek 'iris', meaning 'rainbow',
iridescence is a rainbow-like colour effect
caused by a gem's structure breaking up light
into its spectral colours. With colours that
change depending on the angle of observation,
some everyday examples of iridescence are soap
bubbles and butterfly wings. The 'metallic
iridescence' or 'schiller' in Labradorite it
aptly called 'labradorescence', while the
delicate iridescence seen in Pearls is called
the 'orient' or 'overtone'. Examples of other
iridescent gems are Ammolite, Fire Agate and
Mother of Pearl.
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Labradorite displaying its signature
metallic iridescence |
Play of Colour
Do you remember science class with the prism that
displays the spectral colours of the rainbow? If
yes, you already understand play of colour.
Unique to Opal, 'play of colour' is the flashes
of colour that change with the angle of
observation. An Opal, whose 'play of colour'
flashes are in the rare reds, is generally more
valuable than Opals with just blue and green,
but as with all gemstones, colour preferences
are subjective and should be dictated by
individual tastes.
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Play of colour in Black Opal |
Pleochroism (Double Refraction)
In certain gemstones, atoms are arranged so
that light splits into two separate components.
If this results in optical doubling, then this
is called 'double refractivity' or 'bifringence'.
Zircon is a good example of a doubly refractive
gemstone. If these rays of light possess
different colours visible when the gem is viewed
from different angles, then this is called
'pleochroism' or getting more technical,
dichroism (two-colours, such as Tourmaline) and
trichroism (three-colours, such as Tanzanite).
A common misconception is that double
refractivity makes a gem more brilliant. It
doesn't. In fact, singly refractive gems can
display colours that are purer and more intense
than those seen in doubly refractive gems of the
same colour (e.g. Noble Red Spinel versus Ruby).
Confused? The word that causes the problem is
'double'. While light is being split into two
(doubled), the amount of light exiting the gem
is not.
Pleochroism affects gem buyers in three ways:
the component colours are so similar that
pleochroism usually doesn't affect beauty (e.g.
Ruby and Sapphire); the component colours are different and not all are attractive, so cutters minimise pleochroism by orientating the crystal
to display its single best colour (e.g. Kunzite
and Tanzanite); or the component colours are
different, yet attractive, so the cutters orient
the gem to get a pleasing mix of colours (e.g.
Andalusite). |
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Ratanakiri Zircon is a doubly refractive gemstone |
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Andalusite displaying its pleochroic colours |
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