All gem-quality corundum that isn't 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, however the best is really a clear, deep blue. Some stones, called “colour-change sapphire”, exhibit different shades of blue in artificial and sun light.
Top quality sapphire can be found in Burma, Sri Lanka, and India. The very best Indian Sapphire is cornflower blue, and present in Kashmir, either in pegmatites or as waterworn pebbles in alluvial deposits. Sapphire from Thailand, Australia, and Nigeria is dark blue, and could appear nearly black. Montana (USA) produces sapphire of the attractive metallic blue. Other localities include Cambodia, Brazil, Kenya, Malawi, and Colombia.
Synthetic sapphire production began within the late 1800s. Commercial quantities became available in early 20th century.
Properties Chemical Composition:
Aluminium Oxide - Al
2 O
3 (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
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
Refractive Index / Birefringence:
1.762 - 1.770 / 0.008; Range: 1.754 - 1.778
Pleochroism:
Strong in deep colored varieties
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 450
nm , 460
nm and 470
nm . 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. Simulants (with key 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.
