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Blue Sapphire
Know your SapphireThe second
member of the corundum family-
Sapphire has a great significance in
India as an astrological stone and a
huge following in the west for its
rich royal blue hue. Here we discuss
some features of natural sapphire,
its treated counter parts and
synthetic stones.
Corundum occurs in all colors.
Sapphire is the name given to all
colors of corundum other than red.
When used on its own the name
'sapphire' implies blue sapphire.
All other colors are called sapphire
with the color prefix- for example-
yellow sapphire or green sapphire.
The blue in sapphire is caused by a
combination of titanium and iron.
Some sapphires exhibit a color
change.
OccurrenceSapphire occurs in metamorphic
rocks, lavas and pegmatites however
production is mostly from placer
gravels. Major areas include Sri
Lanka, Australia, Kashmir, Myanmar
(Burma), Thailand, The USA and
Cambodia. Sapphires are also found
in other localities such as China,
Nigeria, Kenya, Tanzania, South
Africa, Mozambique and Madagascar.Inclusions in Natural Sapphire
Color Zoning: Angular zones of a
different shade of color. Either
hexagonal or angular.
Crystal inclusions with stress
cracks: A solid crystal surrounded
by stress cracks either radiating
outwards or surrounding the
inclusion in a 'halo'.
Healed fractures: These are
liquid-filled residual cavities in a
'fingerprint' like healed fracture.
Silk and rutile needles: Elongated,
oriented needle-like inclusions that
cause a milky appearance.
| Localities |
Comments and
Inclusions |
| Kashmir, India |
Restricted production of
fine-colored blue sapphire,
often with a very slight
milky appearance caused by
very fine inclusions. Other
characteristic inclusions
include color zoning, zircon
crystals, stress fractures
and negative crystals. |
| Myanmar (Burma) |
Sapphires of a good
color but sometimes rather
dark. Inclusions may include
long rutile or apatite
needles. Convoluted
feathers, silk, hexagonal
color zoning may be present
in some stones. |
| Kampuchea (Cambodia) |
Medium to good color
stones are found here which
in some cases may appear
similar to Thai stones.
Small red crystals of
different materials may be
found as well as crystals
surrounded by healing
fractures. |
| USA (Montana) |
Untreated blue sapphire is
usually very bright with an
almost metallic luster. They
may appear violet in
tungsten light. Small
well-formed crystals of
garnet, rutile, calcite or
pyrite may be present.
Hexagonal zoning may be
present. Pale green, yellow
and pink colors are also
found here. |
| Sri Lanka |
This
island country is an
important source of good
quality sapphires ranging
from very dark to very pale
blue stones. Some stones are
strongly parti-colored.
Pink, yellow and
pinkish-orange ‘padparascha’
stones do also occur.
Crystal inclusions with
haloes, elongated negative
crystals, healed
finger-print like fractures,
rutile silk and two phase
inclusions are some
characteristic inclusion of
Lankan stones. |
| Australia |
Good
to dark blue, dark
greenish-blue and almost
black sapphires occur here.
Yellow, green and
parti-colored stones are
also common. Strong zoning
may be observed along with
zircon haloes. |
TreatmentMost sapphire is heat-treated to
improve the color. Sometimes it may
be treated by a procedure called
'surface-diffusion treatment' that
imparts color to the outer layer of
the stone. This is done to poor or
colorless material.
To put it in simple terms, Sapphire
heated in an oxidizing atmosphere
lightens its color. Heating in a
reducing atmosphere enhances the
blue color. Of course there are some
conditions and additives needed in
addition to plain heating; however
the main focus here is to identify a
heat treated sapphire.
DetectionColor zoning lines are commonly
poorly defined in heat-treated
stones. Other characteristic
features include cross-hatched
color-zoning and sharply defined
cloudy layers, or patches whose
whitish appearance is caused by
concentrations of submicroscopic
inclusions. In addition to this tiny
inclusions tend to be distributed in
poorly defined stripes and clouds
throughout the stone.
Detection of surface diffusionIn addition to the external features
and modified internal features
described above surface diffusion
treated stones also have the
following features. Re-polishing may
remove more color from some facets
than from others and this may show
up as a patchy effect. Chipping will
also remove areas of color so
chipped areas should be examined
carefully. When immersed in water or
other liquids or when exposed to
diffused light diffusion treated
stones will show a color
concentration along the facet edges
and girdle. The 'bleeding' of color
into surface-reaching fissures and
fractures is also diagnostic.
Detection of Bulk DiffusionIn this recently developed process
corundum is heat treated in the
presence of beryllium. This causes a
diffusion of color through most or
all of the stone. This process was
originally applied to pink sapphires
which turned them into orange
colored stones with a pink center
resembling the expensive 'padparascha'
sapphires.
Immersion of these stones in a
solution of di-iodomethane (methyl
iodide0 may reveal the pink centre
more clearly. However the color of
some stones now permeates the entire
stone and when such a sapphire is
immersed no color difference is
seen.
Synthetic CorundumSynthetic corundum has the same
refractive index and specific
gravity as natural corundum and is
made in all colors by various
methods each of which produces
characteristic inclusions. Some of
these are listed below.
| Method |
Inclusion |
| Verneuil flame-fusion |
Curved growth lines,
gas-bubbles, induced
fractures and healed
fractures. |
| Czochralski pulled |
These stones are
relatively clean but small
gas bubbles may be seen. |
| Floating zone Seiko and
Novosibirsk |
Gas bubbles and
irregular color swirls. |
Flux-melt:
Chatham, Kashan Ramaura,
Knischka Novosibirsk, Douros |
Flux-filled cavities and
healed fractures; tiny flux
particles arranged in
streamer- or comet-like
patterns; platinum
platelets; angular growth
zoning similar to zoning
observed in natural stones.
|
Flux-melt:
Lechleitner overgrowth
|
Seed crystals, generally
with trapped flux on the
boundary but rarely found.
|
| Hydrothermal |
Wavy growth structure,
feathers. |
Detection of Verneuil
Synthetic corundumSome blue stones will show curved
growth lines under low
magnification. If the curved banding
cannot be found either by the
unaided eye or 10 X magnification
against a white background, it may
be visible under a microscope.
Verneiul synthetic corundum may show
elongated bubbles with transverse
constrictions, which are sometimes
referred to as 'proliferated
bubbles'. Stones created thus are
often heat treated and this can
induce realistic looking,
partially-filled fractures
resembling natural 'feathers'. Such
treatment is applied exclusively to
corundum manufactured using the
Verneiul methods and no other
synthetic corundum.
For a conclusive result a laboratory
will determine the absorption
spectra of a particular stone to
give a definitive report.
Synthetic Color Change SapphireThis stone is colored by vanadium
and chromium and is often used as an
alexandrite imitation. It has a
color change with a purplish-red in
tungsten light and bluish-grey to
greenish tinge in daylight. It is
detectable due to its characteristic
absorption spectrum and a lab report
is the best bet to be sure in such a
case.
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