Spinel

Spinel species including red, blue, pink, and colour-change varieties with properties, origins, historical significance, and identification.

By Fabian Moor Last updated
species/spinel red-spinel cobalt-spinel flame-spinel

Introduction

Spinel (MgAl₂O₄) is a magnesium aluminium oxide crystallising in the cubic system;
both the 170 ct Black Prince's "Ruby" in the British Imperial State Crown and the
352 ct Timur "Ruby" are red spinels, confused with corundum until modern gemmology.
[1] Being isotropic, spinel is singly refractive: RI 1.712–1.720 (single
reading, no birefringence), no pleochroism, and no silk inclusions, three immediate
tests separating it from ruby (RI 1.762–1.770, birefringence 0.008, distinct pleochroism).
SG 3.58–3.61, hardness 8 Mohs. [2] Colour spans chromium-red,
chromium-pink, iron-blue, and cobalt-blue; true cobalt-blue spinel confirmed by its
characteristic absorption spectrum is among the most valuable coloured stones. [1]
Spinel is largely free of treatments: heat treatment is rare, oiling and diffusion
are not practised, making it a benchmark for natural untreated colour. Mahenge,
Tanzania, has emerged as the premier modern source for vivid neon-pink and red
specimens rivalling Mogok material in saturation.

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

The most valued spinel colour:

  • Chromophore: Chromium (Cr³⁺) [2]
  • Ideal colour: Vivid red with slight pink or orange modifier
  • Fluorescence: Strong red under UV (like ruby)
  • Distinction from ruby: SR (no birefringence), lower RI, lower SG
  • Trade names: "Balas ruby" (historical)

Blue Spinel

Particularly cobalt-blue specimens:

  • Cobalt blue: Coloured by Co²⁺; vivid, saturated [2]
  • 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

Cobalt Spinel Premium

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"

  • Black Prince's Ruby: 170 ct red spinel in British Imperial State Crown
  • Timur Ruby: 352 ct red spinel with Persian inscriptions; Crown Jewels
  • Samarian Spinel: 500 ct; one of largest known spinels

These stones were identified as spinels only with modern gemology.

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 [1]

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

  • Octahedral crystals: Other spinels or negative crystals
  • Fingerprints: Healed fractures with fluid remnants
  • Swallowtail twins: Characteristic growth pattern [1]
  • Zircon with halos: Common in Sri Lankan material
  • Generally clean: Spinel often has fewer inclusions than corundum

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: [1]

  • 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

References

  1. ↑ 1. Read, P. (2008). Gemmology (3rd ed.). Butterworth-Heinemann. ISBN: 978-0-7506-6449-3. DOI: 10.4324/9780080507224.
  2. ↑ 2. 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.

Related Topics

Physical PropertiesOptical PropertiesCorundum