Corundum
Corundum species including ruby and sapphire varieties, properties, inclusions, origins, treatments, and historical significance.
Introduction
Corundum is aluminium oxide (Al₂O₃) crystallising in the trigonal system, the second-
hardest natural mineral at Mohs 9. As a gemstone it appears in two named forms: red
corundum is ruby, all other colours are sapphire. RI is 1.762–1.770 with birefringence
0.008, easy to read on a standard refractometer and diagnostic against same-colour
spinel (singly refractive, RI 1.712–1.720). SG is 3.99–4.01, distinctly heavier than
spinel (3.58–3.61). Chromium (Cr³⁺) drives ruby's red colour and strong red fluorescence
under long-wave UV; iron and titanium charge transfer produces blue sapphire. [1]
Most market corundum has been heat treated; unheated Mogok rubies showing intact short
rutile silk and strong fluorescence command significant origin premiums. [2]
Kashmir sapphires from the 1881–1887 deposit at 15,000 feet retain the highest sapphire
premiums of any origin due to their velvety "sleepy" cornflower-blue and characteristic
silk. [3] [4] [5]
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⁺). [1] Fine rubies are
among the rarest and most valuable of all gemstones.
Colour and Chromophores
- Colour cause: Chromium (Cr³⁺) substituting for aluminium [1]
- 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 [2][6] | 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
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 [3] | 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 [10]
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 [11]
- 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
Historical Significance
Identification Summary
References
- ↑ 1. Fritsch, E.; Rossman, G. (1987). An Update on Color in Gems. Part 1: Introduction and Colors Caused by Dispersed Metal Ions. Gems & Gemology, 23(3), 126–139. DOI: 10.5741/gems.23.3.126.
- ↑ 2. Palke, A.; Saeseaw, S.; Renfro, N.; Sun, Z.; McClure, S. (2019). Geographic Origin Determination of Ruby. Gems & Gemology, 55(4), 580–613. DOI: 10.5741/gems.55.4.580.
- ↑ 3. Palke, A.; Saeseaw, S.; Renfro, N.; Sun, Z.; McClure, S. (2019). Geographic Origin Determination of Blue Sapphire. Gems & Gemology, 55(4), 536–579. DOI: 10.5741/gems.55.4.536.
- ↑ 4. Zouboulis, E.; Grimsditch, M. (1991). Refractive index and elastic properties of single-crystal corundum (α-Al₂O₃) up to 2100 K. Journal of Applied Physics, 70(2), 772–776. DOI: 10.1063/1.349633.
- ↑ 5. Giuliani, G.; Groat, L. (2019). Geology of Corundum and Emerald Gem Deposits: A Review. Gems & Gemology, 55(4), 464–489. DOI: 10.5741/gems.55.4.464.
- ↑ 6. Hughes, R. (2017). Ruby & Sapphire: A Gemologist's Guide. Lotus Publishing. ISBN: 978-0-9645097-1-9.
- ↑ 7. Nassau, K. (1981). Heat Treating Ruby and Sapphire: Technical Aspects. Gems & Gemology, 17(3), 121–131. DOI: 10.5741/gems.17.3.121.
- ↑ 8. Fritsch, E.; Rossman, G. (1988). An Update on Color in Gems. Part 2: Colors Involving Multiple Atoms and Color Centers. Gems & Gemology, 24(1), 3–15. DOI: 10.5741/gems.24.1.3.
- ↑ 9. Read, P. (2008). Gemmology (3rd ed.). Butterworth-Heinemann. ISBN: 978-0-7506-6449-3. DOI: 10.4324/9780080507224.
- ↑ 10. Shor, R.; Weldon, R. (2009). Ruby and Sapphire Production and Distribution: A Quarter Century of Change. Gems & Gemology, 45(4), 236–259. DOI: 10.5741/gems.45.4.236.
- ↑ 11. Emmett, J.; Scarratt, K.; McClure, S.; Moses, T.; Douthit, T.; Hughes, R.; Novak, S.; Shigley, J.; Wang, W.; Bordelon, O.; Kane, R. (2003). Beryllium Diffusion of Ruby and Sapphire. Gems & Gemology, 39(2), 84–135. DOI: 10.5741/gems.39.2.84.