Corundum
Corundum species including ruby and sapphire varieties, properties, inclusions, origins, treatments, and historical significance.
Introduction
Corundum (Al₂O₃) is one of the most important gem minerals, second only to diamond
in hardness. It produces the world's most valuable coloured stones: ruby and sapphire.
All corundum shares the same crystal structure and fundamental properties, with colour
variations caused by trace element substitutions in the aluminium oxide lattice.
Mineralogy
Crystal System and Structure
- Crystal system: Trigonal (hexagonal division)
- Chemical formula: Al₂O₃ (aluminium oxide)
- Habit: Typically hexagonal prisms, bipyramids, tabular
- Cleavage: None (parting along twin planes)
- Fracture: Conchoidal to uneven
Physical Properties
| Property | Value |
|---|---|
| Hardness | 9 Mohs (defines the scale) |
| Specific gravity | 3.99-4.01 |
| Refractive index | 1.762-1.770 |
| Birefringence | 0.008 |
| Optic character | Uniaxial negative |
| Pleochroism | Distinct to strong (varies with colour) |
| Lustre | Vitreous to adamantine |
Ruby
Ruby is the red variety of corundum, coloured by chromium (Cr³⁺). Fine rubies are
among the rarest and most valuable of all gemstones.
Colour and Chromophores
- Colour cause: Chromium (Cr³⁺) substituting for aluminium
- Ideal colour: "Pigeon blood" - vivid red with slight blue modifier
- Range: Pinkish-red to purplish-red to orangey-red
- Fluorescence: Strong red under UV (due to Cr)
The distinction between ruby and pink sapphire is debated; most authorities
require a certain saturation threshold for the ruby designation.
Major Sources
| Origin | Characteristics | Market Position |
|---|---|---|
| Myanmar (Mogok) | Pigeon blood colour, silk, strong fluorescence | Highest premiums |
| Myanmar (Mong Hsu) | Often dark, frequently heated | Significant production |
| Mozambique | Vivid colours, often large | Major current producer |
| Thailand/Cambodia | Darker, less fluorescent | Historic source |
| Sri Lanka | Lighter pinks, some fine reds | Long history |
| Madagascar | Variable quality | Emerging source |
Sapphire
Sapphire encompasses all non-red corundum, though the term alone typically refers
to blue sapphire. Other colours are called "fancy sapphires" with colour prefix.
Blue Sapphire
- Colour cause: Iron (Fe²⁺/Fe³⁺) and titanium (Ti⁴⁺) charge transfer
- Ideal colour: Vivid, medium-toned blue (violetish-blue to pure blue)
- Kashmir quality: "Velvety" or "sleepy" appearance from micro-inclusions
Fancy Sapphire Colours
| Colour | Chromophore | Notes |
|---|---|---|
| Yellow | Iron (Fe³⁺) | Common; ranges to golden |
| Pink | Chromium + iron | Popular; debates on ruby boundary |
| Orange | Chromium + iron | Rare in pure form |
| Padparadscha | Chromium + iron | Pink-orange; highly valued |
| Green | Iron (Fe²⁺) | Often included |
| Purple/Violet | Chromium + iron/titanium | Beautiful examples exist |
| Colour-change | Vanadium | Blue/violet in daylight; purple/red in incandescent |
Major Sources
| Origin | Characteristics | Market Position |
|---|---|---|
| Kashmir | Velvety appearance, cornflower blue | Highest premiums (historic) |
| Myanmar (Mogok) | Royal blue, good fluorescence | Very high value |
| Sri Lanka (Ceylon) | Light to medium blue, large sizes | Major source; good value |
| Madagascar | Variable colours and quality | Significant production |
| Australia | Dark blue, greenish modifier | Commercial grade |
| Montana | Pastel colours, clean | Collector interest |
Padparadscha
Star Corundum
Star rubies and star sapphires display asterism caused by oriented rutile silk.
Formation
- Rutile (TiO₂) needles oriented along crystal axes
- Three sets of needles at 120° create six-ray star
- Must be cut en cabochon with proper orientation
- Rarely, twelve-ray stars occur (two overlapping six-ray patterns)
Quality Factors
- Star sharpness: Crisp, well-defined rays
- Star centering: Star centred on dome
- Ray completeness: Rays extend fully
- Movement: Smooth, fluid motion with light
- Body colour: Attractive underlying colour
- Transparency: Semi-translucent to opaque
Characteristic Inclusions
| Inclusion | Appearance | Significance |
|---|---|---|
| Rutile silk | Fine needles, often in three directions | Can indicate unheated; dissolved by strong heat |
| Fingerprints | Healed fractures with fluid remnants | Common; useful for origin determination |
| Crystal inclusions | Various minerals (calcite, apatite, zircon) | Help determine origin |
| Hexagonal zoning | Angular colour banding | Follows crystal growth |
| Twinning | Lamellar patterns visible in polariscope | Common; creates parting |
| Negative crystals | Angular voids, may contain fluid | Natural feature |
Treatments
Corundum undergoes various treatments to improve appearance:
Heat Treatment
The most common treatment; improves colour and clarity:
- Low temperature: Reduces silk, improves transparency
- High temperature: Can lighten overly dark stones
- Flux healing: Heals fractures with borax flux
- Prevalence: Majority of market corundum is heated
Detection: Dissolved silk, altered inclusions, stress fractures
Beryllium Diffusion
Beryllium penetrates deep into stone at high temperatures:
- Creates or enhances yellow, orange, pink colours
- Penetration is deeper than earlier Ti diffusion
- Detection requires LIBS or LA-ICP-MS
- Must be disclosed; lower value than untreated
Lead Glass Filling
Fractures filled with lead glass to improve clarity:
- Creates dramatic improvement in heavily fractured rubies
- Easily detected (flash effect, bubbles)
- Durability concerns; low value
- Common in low-end commercial rubies