Gem library
Spinel
The gem long mistaken for ruby
Spinel is gemmology's great injustice: for centuries its finest red specimens were sold and crowned as rubies. The Black Prince's Ruby and the Timur Ruby, jewels of the British Crown, are spinels.
- Hardness (Mohs)
- 8
- Density
- 3.58 – 3.61
- Crystal system
- Cubic
- Refractive index
- 1.712 – 1.736
- Cleavage
- None (imperfect on {111})
Mineralogical identity
- Family
- Oxides — spinel group
- Species
- Spinel
- Variety
- Jedi spinel, Mahenge spinel, cobalt blue spinel
- Chemical formula
- MgAl₂O₄
- Crystal system
- Cubic
- Classification
- Semi-precious stone
- Rarity
- Rare
- Indicative price
- €100 – 5,000/ct (Jedi and cobalt: above €10,000/ct)
Etymology
Probably from the Latin spina, thorn, for its pointed octahedral crystals — or from the Italian spinella.
Physical & optical properties
- Hardness (Mohs)
- 8
- Density
- 3.58 – 3.61
- Refractive index
- 1.712 – 1.736
- Birefringence
- None (isotropic)
- Cleavage
- None (imperfect on {111})
- Fracture
- Conchoidal
- Lustre
- Vitreous
- Diaphaneity
- Transparent
- UV fluorescence
- Strong
- Pleochroism
- None
- Colours
- red, pink, blue, purple, orange, black
Formation, geology & history
A century of belated recognition
The distinction between spinel and ruby was only established in 1783 by Jean-Baptiste Romé de l'Isle, on crystallographic grounds: spinel is cubic and forms octahedra, corundum is trigonal and forms barrel-shaped prisms. Until then, anything red and hard was a ruby, which explains the presence of spinels at the summit of European and Mughal crowns. The Black Prince's Ruby, 170 carats, has adorned the British Imperial State Crown since the fourteenth century; the Timur Ruby, 361 carats, is engraved with the names of the emperors who owned it. Both are Tajik spinels. This late rehabilitation explains spinel's deficit of fame despite its objective qualities.
Remarkable gemmological qualities
Spinel combines advantages: hardness 8, no usable cleavage, a lively lustre, a high refractive index (1.72), and above all exceptional natural clarity. Unlike ruby, it needs no treatment: spinel is one of the very few coloured gems on the market that is almost systematically unheated, unoiled, undiffused. That is a strong argument for collectors tired of treatments. Being isotropic, it has no pleochroism, which gives a constant colour from every angle. Its typical inclusions are small aligned octahedra, sometimes arranged in a «fingerprint».
The rising varieties
The market has changed greatly since the 2000s. The pink-red spinels of Mahenge, Tanzania, discovered in 2007, caused a surge with their neon saturation. Burmese spinels called «Jedi» — a nickname born among dealers for an extreme fluorescent pink-red — exceed €10,000/ct. Cobalt blue spinels from Vietnam and Sri Lanka, coloured by cobalt rather than iron, reach blues worthy of the best sapphire and are extremely rare. Spinel was added to August's birthstones in 2016 by Jewelers of America, an institutional sign of its rehabilitation.
How to recognise it
- IsotropyCubic and therefore without birefringence: between crossed polarisers it stays dark and doubles no edge. Ruby, by contrast, is birefringent.
- Natural octahedraRough crystals form regular octahedra, sometimes twinned into triangular plates (the spinel twin, which gave the twin type its name).
- Absence of treatmentA vivid colour in a very clean stone with no trace of heating is consistent: that is the norm for spinel, unlike ruby.
Imitations, treatments & confusions
- Verneuil synthetic spinelMass-produced since 1908, mostly to imitate other gems (aquamarine, sapphire, zircon) in older costume jewellery. Spherical bubbles and curved striae give it away.
- Sold as rubyA historic error rather than current fraud. The birefringence test settles it: spinel is isotropic, ruby is not.
- Glass and garnetGlass shows bubbles and a soft lustre; garnet has higher density and a different colour range.
Deposits & origins
Burma (Mogok — red spinels and the neon pink-red «Jedi»), Tanzania (Mahenge: pink-red spinels found in 2007), Vietnam (Luc Yen: cobalt blue and violet spinels), Sri Lanka, Tajikistan (Kuh-i-Lal, historic source of the great imperial spinels), Madagascar
Care & precautions
- Warm soapy water and a soft brush; spinel is robust with no usable cleavage.
- Compatible with ultrasonic cleaning and steam on sound stones — a rare privilege among coloured gems.
- Insensitive to sun, salt and common household products.
- At hardness 8 it scratches most other stones: individual storage is advisable.
Traditions & crystal healing
- Chakras
- Varies with colour
- Zodiac signs
- Aries · Sagittarius
- Element
- Fire
- Month
- August (secondary)
Tradition, when it distinguishes spinel from ruby, credits it with renewed energy without excess: a calmer, more sustainable momentum. Black spinels are associated with protection, red ones with vitality, blue ones with clarity. Its history as an overlooked gem also makes it, symbolically, the stone of recognising your own worth.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Is spinel a precious stone?
Legally no: only diamond, ruby, sapphire and emerald carry that title. Gemmologically, its qualities (hardness 8, lustre, absence of treatment) place it level with the best semi-precious stones, and its rare varieties exceed many sapphires in price.
How do I tell spinel from ruby?
Spinel is isotropic (no birefringence, no doubled edges), ruby is birefringent. Spinel is also slightly less dense (3.6 against 4.0).
Is spinel treated?
Almost never, and that is its great asset: virtually all market spinels are natural and unheated, where more than 95 % of rubies and sapphires are treated.