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 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.

Sapphire Ring

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 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.

Treatment

Most 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.

Sapphire diamond ring

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 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.

Blue Sapphire

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 Verneuil Synthetic 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 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.

Sapphire Ring

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 report is the best bet to be sure in such a case.
 
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