Blue Sapphire
by Erum
Sapphire 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.
The 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 itsrich royal blue hue. Here we discuss
some features of natural sapphire,its treated counter parts andsynthetic stones.
Corundum occurs in all colors. Sapphire is the name given to allcolors 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 colorchange.
Occurrence
Sapphire 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 areliquid-filled residual cavities in a’fingerprint’ like healed fracture.
Silk and rutile needles: Elongated,oriented needle-like inclusions thatcause 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. |
Treatment
Most sapphire is heat-treated to improve the color. Sometimes it maybe treated by a procedure called’surface-diffusion treatment’ thatimparts color to the outer layer ofthe stone. This is done to poor orcolorless material.
To put it in simple terms, Sapphireheated in an oxidizing atmospherelightens its color. Heating in a
reducing atmosphere enhances the blue color. Of course there are someconditions and additives needed inaddition to plain heating; however the main focus here is to identify aheat treated sapphire.
Detection
Color 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 diffusion
In 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 Diffusion
In 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 centerresembling the expensive ‘padparascha’ sapphires.Immersion of these stones in asolution of di-iodomethane (methyliodide0 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 Corundum
Synthetic 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 VerneuilSynthetic corundum
Some 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 magnificationagainst 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 areoften heat treated and this can
induce realistic looking, partially-filled fractures resembling natural ‘feathers’. Suchtreatment is applied exclusively to corundum manufactured using the Verneiul methods and no othersynthetic corundum.
For a conclusive result a laboratory
will determine the absorption
spectra of a particular stone to
give a definitive report.
Synthetic Color Change Sapphire
This 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 reportis the best bet to be sure in such a case.
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