Spinel
Spinel species including red, blue, pink, and colour-change varieties with properties, origins, historical significance, and identification.
Introduction
Spinel (MgAl₂O₄) is a magnesium aluminium oxide that produces gems in nearly every
colour. Historically confused with ruby and sapphire, spinel has emerged as a
prized gem in its own right, with fine red and cobalt-blue specimens commanding
exceptional prices.
The "great impostors" of history—the Black Prince's Ruby and Timur Ruby—are
both red spinels, testament to their beauty rivaling ruby.
Mineralogy
Crystal System and Structure
- Crystal system: Cubic (isometric)
- Chemical formula: MgAl₂O₄
- Habit: Octahedra, often twinned
- Cleavage: Imperfect octahedral (rarely seen)
- Fracture: Conchoidal
Physical Properties
| Property | Value |
|---|---|
| Hardness | 8 Mohs |
| Specific gravity | 3.58-3.61 |
| Refractive index | 1.712-1.720 |
| Optic character | Singly refractive (isotropic) |
| Pleochroism | None |
| Dispersion | 0.020 (moderate) |
| Lustre | Vitreous |
Colour Varieties
Red Spinel
Blue Spinel
Particularly cobalt-blue specimens:
- Cobalt blue: Coloured by Co²⁺; vivid, saturated
- Iron blue: Coloured by Fe; often greyish
- Sources: Sri Lanka, Vietnam, Tanzania (cobalt)
- Value: Fine cobalt-blue rivals ruby prices
Pink Spinel
Popular and relatively available:
- Chromophore: Chromium (lower concentrations than red)
- Range: Pale pink to hot pink
- Sources: Myanmar, Tanzania, Vietnam
- Market: Popular in fine jewellery; good value
Other Colours
| Colour | Chromophore | Notes |
|---|---|---|
| Orange (flame spinel) | Cr + Fe | Rare; highly valued |
| Purple/violet | Fe + Cr | Attractive variety |
| Grey/black | Fe (high) | Less valuable |
| Colour-change | Multiple | Blue to purple; rare and valuable |
| Colourless | None | Rare; collector interest |
Major Sources
| Origin | Characteristics | Market Position |
|---|---|---|
| Myanmar (Mogok) | Fine red and pink | Historic premium source |
| Sri Lanka | Various colours; blue notable | Traditional source |
| Tanzania (Mahenge) | Vivid pink and red | Important modern source |
| Vietnam (Luc Yen) | Pink, red, cobalt-blue | Fine material |
| Tajikistan | Red and pink | Historic; limited current production |
| Madagascar | Various colours | Emerging source |
Historical Significance
Spinel has a remarkable history of mistaken identity:
Famous "Rubies"
Historical Importance
- Traded along Silk Road routes for millennia
- "Balas ruby" referred to spinels from Badakhshan (Afghanistan/Tajikistan)
- Prized by Mogul emperors
- Confused with ruby until 18th-century mineralogical advances
Spinel vs Ruby
Spinel
- Singly refractive (isotropic)
- No pleochroism
- RI 1.718 (single reading)
- SG 3.60
- Often cleaner than ruby
- No silk inclusions
- Octahedral crystal habit
Ruby
- Doubly refractive
- Strong pleochroism
- RI 1.762-1.770 (two readings)
- SG 4.00
- Inclusions common
- Silk (rutile needles) characteristic
- Hexagonal prism/bipyramid habit
Characteristic Inclusions
Spinel inclusions help separate natural from synthetic:
Natural Spinel Inclusions
Synthetic Spinel Indicators
- Gas bubbles: Curved lines or scattered
- Strong ADR: More pronounced than natural
- Unusual colours: Especially colourless, blue (as diamond simulant)
- Too perfect: Lack of natural features
Anomalous Double Refraction
Treatments
Spinel is notable for being largely untreated:
- Heat treatment: Rare; occasionally used to lighten dark stones
- No oiling or filling: Not practiced
- No diffusion: Not known in spinel
- Market position: "Untreated" is standard expectation
This untreated status adds to spinel's appeal for collectors.
Identification Summary
Key features for spinel identification:
- RI: 1.718 (single reading—isotropic)
- SG: 3.60
- Optic character: SR (stays dark in polariscope, or shows ADR)
- No pleochroism: No colour change in dichroscope
- Spectrum: Cr doublet at 686nm in red spinel
- Fluorescence: Strong red in Cr-coloured stones