|Chemical composition||Mg(Al2O4) Magnesium aluminum oxide|
|Habit||Octahedral, contact twins|
|Refractive index||1.712 - 1.736|
|Specific gravity||3.58 - 3.61|
Spinel is a mineral species. For many centuries, most gem spinels were misidentified as sapphire or ruby because they have similar properties and occur in the same geological deposits. The historically significant 5.08 centimeter "Black Prince Ruby" in the center of the British Imperial Crown was only recently identified as a spinel. This stone is irregular in shape and has a somewhat squareish outline. Additionally, it was not faceted, merely polished. Spinels also occur in a vast array of colors. They are slightly softer than sapphires but still very durable.
The earliest known use of spinels was as ornaments found in Buddhist tombs in Afghanistan. Blue spinels have been found in England, dating back to the Roman occupation (51 BC to 400 AD).
Common spinel belongs to the "spinel series", which belongs to the "spinel group".
The general formula for the spinel group is A2+B3+2O4. The 3 series of the spinel group are defined by the B3+ cation.
The spinel group is made up of 3 isomorphous series.
The isomorphous series:
- Spinel series (aluminum)
- Spinel - MgAl2O4 (n = 1.719, sg ~ 3.60)
- Hercynite - FeAl2O4
- Gahnite - ZnAl2O4 (n = 1.805, sg = 4.62)
- Galaxite - MnAl2O4
- Magnetite series (ferric iron)
- Magnetite - FeFe2O4
- Magnesioferrite - MgFe2O4
- Ulvöspinel - FeFeTiO4
- Franklinite - ZnFe2O4
- Jacobsite - MnFe2O4
- Trevorite - NiFe2O4
- Chromite series (chrome)
- Chromite - FeCr2O4
- Magnesiochromite - MgCr2O4
Most of the above series members are rare in nature with the exception of the members of the spinel series, magnetite and chromite. To gemologists common spinel and gahnite are of most interest.
When gemologist refer to "spinel", we usually imply common spinel, that is the spinel that belongs to the spinel series of the spinel group.
Spinel can be confused with many stones by appearance alone, yet optical properties usually rule out most of them.
As spinel belongs to an isomorhous series, the optical and physical properties may vary.
Spinel: colorless, green, blue, red, black.
Gahnite: blue-green, yellow, brown.
- Pleonaste (also named ceylonite) - (Mg,Fe)Al204 - dark green to blue-green; black
- Gahnospinel - (Mg,Zn)Al204 - pale to dark blue
Spinel is allochromatic and variety colors are produced by transition metals:
- Cr3+ - red, pink
- Fe2+ - blue, violet
- Fe2+ + Co2+ - darkblue
- Fe3+ - green
Transparent to opaque.
Spinel is isotropic and the refractive index of common spinel is generally around 1.712 to 1.720. Red spinel can have a refractive index up to 1.74.
The only other isotropic gemstone that falls within this range is grossular garnet, but it will usually be higher and the color is also different.
Other members of the spinel series, such as gahnite will have higher refractive indices.
All other gemstones can easily be seperated from spinel by their optic nature.
Synthetic spinel has a usual refractive index of 1.726.
Gahnospinels have a refractive index between 1.725 and 1.753, while pleonaste (ceylonite) has an R.I. range of 1.77 to 1.80.
As with the refractive indices, the specif gravity of spinel can vary due to isomorphous replacement.
The values for most gem grade material lies between 3.58 and 3.61. Pleonaste has a S.G. between 3.63 and 3.90.
Gahnospinels may have a specific gravity up to 4.06.
Common spinel is isotrope and will remain dark under crossed polars.
Verneuil type synthetic spinel will always (maybe with the exception of red) show strong anomalous birefringence due to excess Al2O3 (see synthetics). This anomalous extinction (as it is currently named) can be seen as "tabby" extinction, resembling the color distribution of a cat's fur, or/and as an Andreas cross.
Natural and flux-melt synthetic spinel may show weak anomalous extinction.
Spectrum of a natural blue spinel, colored by iron.
- Asterism (4 and 6-pointed stars)
- Color change (rare)
Myanmar; Vietnam; Thailand; Sri Lanka; Pakistan; Afghanistan; Tadshikistan; Kenia; Tanzania; South-Africa; Brazil.
Spinel is synthesized by the Verneuil (flame-fusion) process and the flux-melt method, although the first process does not render a true synthetic in most cases.
Flame fusion synthetic spinels are produced since 1908 (by accident while creating synthetic corundum), but were not commercially available until 1930.
It was found that while trying to synthesize spinel through the Verneuil process, the resulting boules would easily fracture and no reasonably sized gemstones could be cut from them.
The ratio MgO to Al2O3 is 1:1 for common spinel and by changing that ratio (adding Al2O3) the boules became stable. Because that alters the chemical formula of the synthetic, it is not a true synthetic (but accepted as such). Sometimes these are named "beta-corundum" due to the excess of alumnia.
For the creation of red stones, this alteration was no option and, usually, no red synthetic spinel boules created by the flame-fusion process result in large stones (but are known to excist). The larger sized red synthetic spinels are mostly created with the flux-melt method. The few red synthetics that are created through the Verneuil process will show curved striae like their synthetic corundum cousins.
As a result in the changing of the MgO:Al2O3 ratio, the flame fusion synthetics have higher refractive indices (usually stable at 1.726) and a higher specific gravity (3.64).
|Properties of flame fusion synthetic spinel|
|color||coloring agent||RI||SG||other diagnostics|
|colorless||none||1.728-1.740||3.65-3.80||LWUV: green; SWUV: white/blue|
|red||Cr3+||1.722-1.725*||3.58-3.60||curved striae; spectrum; fluorescence|
|emerald green||Mn + Co3+||do.||do.|
|beryl green||Cr3+ + Mn||do.||do.||spectrum|
|zircon blue||Co3+ + Cr3+ + Ti||do.||do.||spectrum|
|sapphire blue||Co3+||do.||do.||spectrum; flourescence|
|amethyst violet||Co3+ + Mn||do.||do.|
|alexandrite color change||Cr3+ + V||do.||do.||spectrum|
Strong "tabby" extinction can be seen in all
First commercial production of flux-melt synthetic spinel started around 1980, although successful experiments date back to 1848 (by the French chemist Ebelmen).
Apart from red, blue synthetics have also been produced by the flux-melt process.
These are true synthetics.
Czochralski pulling method
A recent development (2007) is the procuction of red to pink synthetic spinels by the Czochralski pulling method.
- Introduction to Optical Mineralogy 2004 - William D. Nesse ISBN 0195149106
- Gemmology 3rd edition (2005) - Peter G. Read ISBN 0750664495
- Gems, Their Sources, Descriptions and Identification 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)
- Edelsteinkuntliches Praktikum - Ulrich Henn, Gemmologie Jahrgang 44 / Heft 4 / Dezember 1995, Spinell pp.54-62