SPINEL   redspinelcropped.gif (3066 bytes)

   Spinel (MgAl2O4) is a much maligned gem mineral that is often thought of as a corundum simulant because in red and blue varieties it can closely resemble ruby or sapphire. It is, however, an outstanding gem material in its own right; it is hard (8), possesses an intermediate to high R.I., shows good dispersion, and is available in a wide variety of colors. Many of the worlds most famous large "rubies" are, in fact, red spinel (e.g. Black Princes' Ruby and Timur Ruby, both in British Crown Jewels). Such stones were once referred to as Balas Ruby, a term that is not used today but is synonymous with red spinel. The confusion is well-founded; the finest red spinels come from the same gem gravel deposits in Myanmar that are renowned for "Burma" ruby, and both share somewhat similar optical and physical properties.

    Spinel, like garnet and tourmaline, is a mineral name that refers to a group of minerals all having the same crystal structure. Members within the group differ by containing varying amounts of Fe, Cr, Zn, Mn and Ni that substitute for Mg and Al in the crystal lattice. Probably the most familiar minerals that have the spinel crystal structure are the ore minerals magnetite (Fe2+Fe3+2O4) and chromite (Fe2+Cr2O4), neither of which is a gem material. Gem spinel is usually quite close in composition to Mg, Al spinel but contains small amounts of Fe, Zn, and Cr that act as chromophores and account for the wide range of possible colors. Red and pink spinel is colored by trace quantities of Cr; blue, violet, orange and green by Fe. Zn, Cr, and Co (rare) are also present in some blue, violet and purple stones.

    "Spinel" is synthesized in the lab by the flame fusion process (Verneuil method) and is widely used as a corundum, diamond, and chrysoberyl imitation. Virtually any color of the rainbow can be made, including those not found in nature (yellow), but all differ from natural spinel in chemical makeup (more Al than Mg in comparison to naturals). This difference is reflected in the R.I. and S.G., which are consequently valuable properties in distinguishing naturals from synthetics, or synthetic spinel from corundum (see below).


Properties

  • Crystal System: Isometric (Cubic)
  • Crystal Habit: Occurs as octahedra or as flat triangular plates that result from twinning ("spinel twins").
  • Hardness: 8
  • Cleavage: none
  • Fracture: conchoidal
  • Toughness: good
  • Specific Gravity: varies with composition; most gem varieties in the range 3.58-3.61. Zn-rich spinel (rare) can be as high as 4.40. Synthetic spinel has an S.G. of 3.63-3.64.
  • R.I.: about 1.72 for most natural gem material; red spinel may be as high as 1.74, Zn-rich as high as 1.805. Synthetic spinel has a noticeably different R.I. of 1.727+0.001.
  • Birefringence: none, isotropic. Synthetic spinel often shows a patchy A.D.R..
  • Dispersion: medium, 0.02
  • Color: colorless, pink, red, lavender, blue, green, brown, black
  • U.V. Fluorescence: strongest under long wavelength u.v., weak to absent in short wavelength u.v.. Color varies with body color:
    • Red and pink: crimson
    • Purple to mauve: red or orange
    • Dark Blue: none
    • Violet-blue to pale blue: weak pale green
  • Synthetic spinel may often show a bright blue, chalky green, or red fluorescence in short wavelength u.v.light.

Distinguishing Properties

  • Natural spinel can be distinguished from corundum using a polariscope; synthetic spinel cannot because it often shows anomalous double refraction (A.D.R.; see garnet notes). R.I. and S.G. are useful in these situations.
  • S.G. and R.I. can overlap with pyrope garnet; can be distinguished in these cases by fluorescence and careful study of inclusions.
  • From synthetic spinel by R.I., S.G., fluorescence and inclusions. Synthetic spinel has an R.I. of about 1.727 and an S.G. of 3.63-3.64. Natural spinel does not fluoresce strongly under short wavelength u.v., whereas some synthetic spinels do.
  • Synthetic dark red spinel is apparently not manufactured in the quantities that most other colors are. Most synthetic dark red stones are synthetic corundum.
  • Synthetic spinel that shows a color change from violet in daylight to a more reddish color in incandescent light is sold in some countries and by disreputable dealers as Alexandrite, which it in no way resembles. Natural alexandrite is a variety of the mineral chrysoberyl, and shows a color change from green to red. Trade names for this material include "Alexandrium".

Sources

    Gem spinel forms in the same rocks and by the same processes that produce gem corundum. The most common occurrence of gem spinel is in metamorphosed impure limestones (marble), but it has also been found in pegmatites (Zn-rich varieties, these are rare). The principle historical producer of spinel was Burma (Myanmar). Presently most production is from the gem gravels of Sri Lanka and Thailand.

  • Burma (Myanmar); Mogok Stone Tract
    • Source of most of the famous large red spinels; finest red and pink spinel found here.
    • Source is metamorphosed limestone and its weathering products (gem gravels).
    • Grey-blue or black material may show 6- or 4-rayed stars.
    • Magnetite inclusions are characteristic.
  • Sri Lanka, Thailand, Cambodia
    • Same gem gravels that produce corundum.
    • Most Sri Lankan spinels are said to be paler (color "more subdued"; "lighter tone") and less included than their Burmese counterparts.
  • Pakistan, Karakoram Range
    • In marble
    • Reds, brownish to plum-red, lilac, violet, and blue.
    • Contain inclusions of green amphibole and rutile.
  • Others: Nigeria, Cambodia, Afghanistan. There are reports of a recent find of a large deposit of fine pink spinel in the Pamir Mts., Tajikistan.

Shaping and Treatment

  • Step and brilliant cuts. Lack of cleavage and pleochroism make these relatively simple stones to cut.
  • Have not yet found any reference to heat treatment or irradiation to change color. Those containing Fe could be treated; see sapphire notes for possibilities.

Pricing and Valuation

    Little info.. As with other colored stones, depth and purity of color and clarity are highly prized. Reds and pinks are apparently the most expensive, closely followed by lavender and blues. The price trend with color seems to follow sapphire. Good gem spinel in any color (particularly pinks and reds) in sizes of 3 carats or above is very rare (rarer than ruby or sapphire). Several authors state that given the rarity and beauty of good pink and red spinel these stones are highly undervalued and should continue to increase in price. Prices have historically suffered from confusion and comparison with corundum and synthetic spinel. Detractors from value are brownish or Grey overtones, or a muddy color, particularly in reds and blues.

    Prices for 3-5 carat or stones of top quality where in the following ranges in 1990:

  • Blue - $100-300/ct; no gray or other overtone.
  • Red - $500-$1500/ct.; no brown or gray overtone; should resemble fine ruby.
  • Pink - $100-$600/ct
  • Lavender - $50-$350/ct

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1998
Updated 08/20/09
Comments and questions to helper@mail.utexas.edu
Department of Geological Sciences
The University of Texas at Austin