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Sapphire

Unknown16.12 |
All gem-quality corundum that is not red is known as sapphire, yet this name is popularly linked to the colour blue. Variation in colour, because of iron and titanium impurities, spans many shades, but the most valuable is clear, deep blue. Some stones, called colour-change sapphire, exhibit different shades of blue in artificial and natural light.

Good quality sapphire is found in Burma, Sri Lanka, and India. The best Indian sapphire is cornflower blue, and found in Kashmir, either in pegmatites or as waterworn pebbles in alluvial deposits. Sapphire from Thailand, Australia, and Nigeria is dark blue, and may appear nearly black. Montana (USA) produces sapphire of an attractive metallic blue. Other localities include Cambodia, Brazil, Kenya, Malawi, and Colombia.

Synthetic sapphire production began in the late 19th century. Commercial quantities became available in the early 20th century.

Sapphire Properties

Chemical Composition: 
Aluminium Oxide - Al2O3 (trace elements Chromium, Iron, Titanium, etc. are responsible for the different colored varieties.
Colors / Varieties: 
  • White Sapphire
  • Blue Sapphire
  • Pink Sapphire
  • Green Sapphire
  • Yellow Sapphire
  • Pink-Orange Sapphire (Padparadscha)
  • Phenomenal
    • Color Changing Sapphire
    • 6-Ray Star Sapphire
    • Parti-colored Sapphire
    • Trapiche Sapphire
Crystal System / Forms: 
Trigonal System
Hardness: 
9
Specific Gravity: 
3.98 - 4.00
Cleavage / Fracture: 
None may show parting / Conchoidal fracture. (Parting or false cleavage occurs along directions of weakness which are parallel to the basal plane or the rhombohedral faces of the crystal and are observed externally as grooves or striations on the surface.
Optic Character: 
Anisotropic, D.R., Uniaxial negative
Lustre: 
Vitreous
Refractive Index / Birefringence: 
1.762 - 1.770 / 0.008; Range: 1.754 - 1.778
Pleochroism: 
Strong in deep colored varieties
Dispersion: 
0.018
Magnification: 
  • Crystals of calcite, dolomite, apatite, etc.
  • Silk - three directional needles of rutile intersecting at 60° / 120°; Needles of boehmite intersecting at almost 90°; Needles of ilmenite etc.
  • Parallel polysynthetic twin lamellae.
  • Color zoning and angular growth zoning.
  • Fingerprints and fluid inclusions.
  • Zircon halo inclusions etc.
U.V. Fluorescence: 
Varies according to the color.
Spectrum: 
Blue, green, brown and yellow sapphires: shows a three band absorption in the blue at 450nm, 460nm and 470nm. Generally at least one of these bands is present.
Cause of Color: 
  • Blue Sapphire: Traces of Iron and Titanium.
  • Green / Yellow Sapphire: Traces of Iron (Fe2+ / Fe3+).
  • Color Changing Sapphire: Traces of Chromium and Vanadium.
Treatment (Enhancement): 
  • Coating: To improve the color.
  • Colorless Impregnation: With oils and glass (silica / lead).
  • Colored Impregnation: Of lighter colored sapphires to enhance the overall color.
  • Heat Treatment: To darken or to lighten the color; to induce asterism, etc.
  • Irradiation: To darken the color and improve clarity in yellow sapphires, but color may fade with time / heat.
  • Diffusion Treatment (Surface / Beryllium): Color enhancement. Improvement of star stones.
Synthesis: 
  • Verneuil Method: Sapphire, Star Sapphire
  • Czochralski: Sapphire, Star Sapphire
  • Flux Fusion Method: Sapphire
  • Hydrothermal Method: Sapphire
Simulants (with separation tests): 
  • Blue Sapphire: Tanzanite (optic figure, pleochroism, R.I., S.G.), Natural / Synthetic Spinel (optic character, pleochroism, R.I., spectrum, S.G.), Benitoite (dispersion, doubling), Iolite (optic figure, pleochroism, R.I., S.G.), Kyanite (optic figure, R.I., S.G., inclusions), Synthetic Quartz (optic figure, R.I., S.G.), Glass (optic character, inclusions), Doublets (inclusions, U.V. fluorescence).
  • Yellow Sapphire: Chrysoberyl (optic figure, spectrum, inclusions), Citrine (optic figure, R.I., S.G., inclusions), Topaz (optic figure, R.I., S.G.), Yellow Beryl (R.I., S.G., inclusions), Hessonite Garnet (optic character, inclusions), Sillimanite (optic figure, inclusions, R.I., S.G.), Synthetic Cubic Zirconia (dispersion, S.G., U.V. fluorescence), etc.
Geological Occurrence: 
In igneous and metamorphic rocks (marble, dolomite, etc.
Sources: 
India (from Kashmir as Kashmiri Sapphire, Karnataka, Orissa), Myanmar (known as Burmese Sapphire), Thailand, Sri Lanka, Australia, Africa (Tanzania, Nigeria, Kenya, Madagascar), U.S.A. (Montana), Cambodia (Pailin).
Cuts & Uses: 
Facetted cuts (usually mixed cuts), cabochons, beads, carvings, etc.