Spinel

The name spine has been derived from the Latin word spina, meaning little thorn, referring to the sharp points on some crystals. Sometimes blood red spinel is known as ruby spinel.

Spinel is found in a wide range of colors due to the presence of various impurities, and is transparent to almost opaque. Red spinel is due the presence of chromium + iron and is the most popular, although for many years it was thought to be a variety of ruby. The orange-yellow or orange-red variety is called rubicelle (a diminutive of the French word for ruby). Blue spinel is due to the presence of iron and less commonly by cobalt. Occasionally, inclusions of crystals such as magnetite or apatite may occur, and some Sri Lankan spinels may include zircon crystals surrounded by brown haloes. Star stones are rare, but when in cabochon may show 4-rayed or 6-rayed starts.

Spinel occurs in granites and metamorphic rocks, and is often found in association with corundum. Octahedral crystals and waterworn pebbles in a wide range of colors are found in the gem gravels of Burma, Sri Lanka, and Madagascar. Other localities include Afghanistan, Pakistan, Brazil, Australia, Sweden, Italy, Turkey, the former USSR, and the USA.

Synthetic spinel has been manufactured since 1910. It has been used to imitate diamond, or colored to imitate gemstones such as aquamarine and zircon. Blue synthetic spinel, colored by cobalt, has been used to imitate blue sapphire.

Cause of Color :
  • Red / Pink: Chromium
  • Blue: Iron and / or cobalt
Chemical Composition :

Magnesium Aluminium Oxide – MgAl2O4

Isomorphous replacement of

  • Magnesium by Fe2+ or Mn, or wholly by Zn
  • Aluminium by Fe3+ or Cr

Gahnospinel – (Mg,Zn)Al2O4

Classification / Type :

Spinel belongs to an isomorphous series with a number of species. Of gem interest are basically spinel and gahnospinel

Crystal System / Forms :

Cubic System / Octahedral crystals; contact twining very common. Triangular etch marks on octahedral face

Cuts & Uses :

Facetted cuts, cabochons, beads, carvings, etc.

Dispersion : 0.020
Hardness : 8
Lustre : Vitreous
Magnification :

Crystals (calcite, apatite, etc), octahedral crystals (singly or in a finger print); two or three direction silk needles, iridescent cleavage fractures, iron stained films, negative crystals, crystals with stress fractures.

Optic Character :

Isotropic, S.R.

Pleochroism : None.
Refractive Index / Birefringence :

Generally 1.718. Range: 1.710 – 1.750 with increase in Cr%. Gahnospinel: 1.73 – 1.80

Simulants (with separation tests) :

Natural / Synthetic Corundum (optic character, inclusions, pleochroism), Kyanite (optic character, inclusions), Pyrope Garnet (spectrum, U.V. fluorescence, inclusions), etc.

Sources :

Sri Lanka, Myanmar (Burma), India, Tanzania (Umba valley), Vietnam, Afghanistan, etc.

Spectrum :
  • Red / Pink Spinel: Fine, fluorescent lines between 650nm and 700nm (known as Orange Pipe spectrum, best visible with blue filter), broad band centered at 540nm, other lines at 640nm and 470nm may be seen.
  • Blue Spinel: Strong band at 458nm, other weak bands at 478nm, 508nm, 555nm, 585nm, 635nm.
  • Cobalt Blue Spinel: Strong bands centered at 540nm, 570nm, 630nm and 458nm.
Synthesis :
  • Synthetic Spinel by Flame Fusion: S.G. 3.61 – 3.65, R.I. + 1.73, strong A.D.R. effect, chalky under shortwave, gas bubbles and curved lines under magnification. Synthetic cobalt blue spinel: strong bands centered at 540nm, 570nm and 630nm.
  • Synthetic Spinel by Flux Fusion: flux fingerprints, metallic platelets. Synthetic red spinel exhibits strong purplish to orange-red under ultra violet longwave.

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