Ruby - the name given to red, gem-quality corundum - is one of the best gemstones for jewellery settings. Rubies may be any shade of red, from pinkish to purplish or brownish red, depending on the chromium and iron content of the stone. Frequent twinning of the crystals makes the material liable to fracture, yet ruby is a tough mineral, second only to diamond in hardness. Crystal prisms are hexagonal with tapering or flat ends. As the crystals grow they form new layers, and depending on the geological conditions and minerals present, colour variations, called zoning, occur.

Worldwide in igneous and metamorphic rocks, or as waterworn pebbles in alluvial deposits. The finest ruby comes from Burma; those from Thailand, the main source, are brownish red; Afghanistan, Pakistan and Vietnam produce bright red stones; those from India, North Carolina (USA), Russia, Australia, and Norway are dark, sometimes even opaque. In 1902, a Frenchman, Auguste Verneuil, produced a synthetic ruby crystal by exposing powdered aluminium oxide and colouring material to the flame of the blowtorch.

Chemical Composition: 

Aluminium Oxide - Al₂O₃ (trace elements Chromium, Iron, Titanium, etc. are responsible for the different colored varieties

Colors / Varieties: 

Red / Reddish-pink

• 6-Ray Star Ruby
• Trapiche Ruby

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: 

Strong absorption of the yellow, green and violet wavelengths. A broad band centered at around 550nm, a series of fine lines in the red, two of which appears as a single line which is actually 'doublet' at 694.2nm and 692.8nm (two lines closely spaced) which in appropriate lighting are fluorescent. Other lines are at 468nm, 475nm and 476.5nm. The presence of Iron in some rubies suppresses the fluorescence and imparts a duller color to the stone.

Cause of Color: 

Chromium, Iron (may / may not be present).

Treatment (Enhancement): 

• Coating: To improve the color.
• Colorless Impregnation: With oils and glass (silica / lead).
• Colored Impregnation: Of lighter colored rubies to enhance the overall color.
• Heat Treatment: To remove silk inclusions and improve clarity; to remove the secondary (blue / violet / brown) color component; to induce asterism, etc.
• Diffusion Treatment (Surface / Beryllium): Color enhancement. Improvement of star stones.

Synthesis: 

• Verneuil Method: Ruby, Star Ruby
• Czochralski: Ruby, Star Ruby
• Flux Fusion Method: Ruby
• Hydrothermal Method: Ruby

Simulants (with separation tests): 

Natural / Synthetic Spinel (optic character, R.I., inclusions), Tourmaline (R.I., S.G., pleochroism, inclusions), Red Garnets (optic character, spectrum), Glass (optic character, inclusions, U.V. fluorescence), Glass (optic character, inclusions, U.V. fluorescence), Doublets (inclusions, U.V. fluorescence), Synthetic Cubic Zirconia (optic character, R.I., S.G.), etc.

Geological Occurrence: 

In igneous and metamorphic rocks (marble, dolomite, etc.

Sources: 

Myanmar (known as Burmese Ruby), Thailand, Sri Lanka, Africa (Kenya, Tanzania, Madagascar), India (Orissa, Kangyam - Kaur, Karnataka, Andhra Pradesh, Madhya Pradesh), Vietnam.

Cuts & Uses: 

Facetted cuts (usually mixed cuts), cabochons, beads, carvings, etc.