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The Splendour of Spinel, part 2: Types and Treatments

Spinel is one of many gemstones for which there are numerous varieties. Just like its cousin corundum, spinel gems can occur in a myriad of different hues

Very much like rubies and sapphires, spinel is capable of exhibiting nearly every colour of the rainbow when the right trace elements are present as impurities within its crystal structure. Red and pink hues in spinel are caused by the presence of trivalent chromium (Cr3+) impurities, while blue hues are caused by the presence of ferrous iron (Fe2+); in some cases trivalent cobalt (Co3+) may contribute to the blue colour of spinel and even induce blues as vibrant as some of the highest quality blue sapphires, as is the case with aptly named “cobalt spinel” gemstones. Lavender and purple are among some of the more common colours displayed by spinel and these are typically the result of both trivalent chromium (Cr3+) and ferrous iron (Fe2+) being present. Orange spinels are sometimes found among red spinel crystals, with these stones bearing both trivalent chromium (Cr3+) and yellow chromophores; it is not definitely clear what ions act as yellow chromophores in spinel, but ferric iron (Fe3+), iron-to-iron charge transfers (Fe2+-Fe3+), and divalent manganese (Mn2+) have all been proposed as possible sources for yellow hues in spinel gems. Yellow and green spinels do exist, but these are exceptionally rare, with yellow spinel bearing only yellow chromophores (Fe3+, Fe2+-Fe3+, and/or Mn2+) and green spinel bearing a mixture of yellow chromophores (Fe3+, Fe2+-Fe3+, and/or Mn2+) and blue chromophores (Fe2+ and/or Co3+)

 

Four examples of different hues which spinel can potentially display;

The least valuable and perhaps most common colour exhibited by spinel is an opaque black; despite their lower value, black spinels make for an attractive and significantly less costly alternative to black diamonds due to their high hardness and sub-adamantine lustre, which give them both a similar appearance and functionality in rings and other jewellery pieces.. The dark opaque appearance of black spinel is thought to be the result of magnesium oxide (MgO) impurities, although zinc oxide (ZnO) has also been proposed to contribute to the colouration of some black spinel gemstones. Intriguingly, spinel is capable of being coloured by these compounds without appearing completely opaque, such is the case for the rather peculiar “grey” spinel variety. Gems of this kind are neither colourless nor black, but they exhibit a shadowy colouration which some industry members compare to that of many metals; due to this, grey spinels are frequently marketed under the terms “silver spinel”, “platinum spinel”, and “steel spinel”. Grey spinel gemstones which are entirely grey and show no coloured undertones are rather rare, with most grey spinels exhibiting very minor hints of pink/purple or blue.

A grey spinel gemstone; Image: Robert Weldon/ GIA
A pair of gold earrings featuring black spinel briolettes; Image: Gemstonist

What is perhaps the most valuable variety of spinel is in fact a sub-variety of red spinel known as “Jedi spinel”. This form of red spinel was named after the monastic “Jedi Order” featured in the Star Wars universe due to its exceptionally strong red fluorescence, which is so bright that some have described these spinel crystals as being “untouched by the dark side”, a description which could be applied to the Jedi of Star Wars that is equally fitting for a spinel gemstone, as dark tones in a spinel diminish their beauty and thus their value. Interestingly, the glowing red character of Jedi spinel is also reminiscent of the glowing red lightsabers seen in the Star Wars films, however these red bladed weapons are wielded by the “dark side” aligned “Sith Order” rather than by members of the Jedi Order; although slightly more intuitive, the name of “Sith spinel” may have been less attractive and less accessible to the average buyer. The first spinel stones to be known as “Jedi spinel” were originally found at Namya in the Mohnyin District of Myanmar’s Kachin State following a mining rush that occurred in the area during the year 2000. Unfortunately the production from this find was limited and very little Jedi spinel made it to market. Some time after this, Jedi spinels were found within the Mogok Valley in the Pyin-Oo-Lwin District of Myanmar’s Mandalay Region, a mining district rich in spinel, corundum, Peridot, and other coloured gemstones. These new gems were coming from an area known as “Mansin” which is now the World’s primary source for Jedi spinel. The production from the Mansin mine was notably more fruitful than that of the original Namya find, and as a consequence the distribution of Jedi spinels into the marketplace was broad enough to make an international impact and give this gemstone the legend-like status by which it is known today.

“Jedi” spinel crystals from Mansin, Myanmar; Image: Vincent Pardieu/ GIA
“Jedi” spinel crystals from Mansin, Myanmar (left) compared to standard red spinels from Mogok, Myanmar (right); Image: Vincent Pardieu/ GIA

Treatments are very rarely applied to spinel gemstones but treated spinel gems are not unheard of. A very small number of pink and red spinels on the market have been given heat treatment. Enhancing spinel gems with heat improves the stones’ clarity but it darkens their colour; magnesium (Mg) and aluminum (Al) atoms are disorganised by the heating process causing a shift in the fluorescence spectrum of chromium, which diminishes the apparent beauty of red and pink spinel, yet this shift allows for heat treated stones to be easily identified in a laboratory. Spinel gemstones may also undergo diffusion treatment, responding to this form of enhancement in a similar manner to diffusion treated sapphires. In the case of spinel gems, the diffused chromophore is typically cobalt, giving the spinel a rich blue colouration. Diffusion treated sapphires are often easy to identify: under immersion higher concentrations of diffused chromophore can be observed near the surface of a gemstone and in healed fissures, with facet junctions typically giving the best view of these added colour zones. Unfortunately the cobalt diffusion treatment applied to some spinel gems can penetrate much deeper into the stone than the titanium diffusion and beryllium diffusion treatments which are typically used for blue sapphires, and consequently this makes identification of diffusion treated spinel gemstone much more difficult.

A red spinel gemstone from Tanzania (left) which has undergone heat treatment (right); Image: GemGuide/ GIA
A blue spinel gemstone which has undergone cobalt diffusion treatment (left) shown in cross section (right). Note the depth at which diffused colour can be observed within the gem; Image: GemGuide/ GIA

Just like corundum, spinel can be produced using laboratory methods. Today, both flame fusion and flux grown spinel gems are available, with such stones frequently appearing in the same jewellery designs which often feature synthetic corundum gemstones, like class rings and various costume jewellery pieces. The flame fusion method for producing spinel was the first to be developed, originally being introduced in the early 20th century in conjunction with the introduction of flame-fusion grown corundum gems. At one point in time, synthetic spinel was very popular among lab-grown gemstones, especially those used in rings, but in recent years synthetic corundum has become much more dominant in the jewellery industry.  

Synthetic spinel crystals (left and right) and faceted synthetic spinel gemstones (right of centre); Image: Robert Weldon/ GIA

 

Among the rarer varieties of spinel there exist gemstones which display optical phenomena. One such variety is colour-change spinel, which exhibits a shifting body colour under different lighting conditions in a manner similar to that of colour-change sapphires. The phenomenon is attributed to variable quantities of trivalent vanadium (V3+) and trivalent chromium (Cr3+) impurities which give off different amounts of red wavelengths under different lighting conditions. This causes the amount of red colouration seen in a gem to increase when moved from natural light to warm-toned light, like that of incandescent bulbs or flames, which then leads to an apparent change in a gem’s hue. Spinel may also exhibit the phenomenon of asterism in a manner which closely resembles that of star sapphires and star rubies. Much like star corundum gems, star spinels have three sets of crystallographically oriented needle-like rutile crystals within them which give off a chatoyant reflection of light when the stone is cut en cabochon; since three sets of inclusions are present, three different cat’s eye effects are produced, which then intersect and resemble a six-ray star dancing across the surface of the gemstone if viewed under non-diffuse direct lighting conditions, although four-ray stars have also been observed in spinel gems. 

A colour-change spinel gemstone from Sri Lanka, showing blue hues when viewed in daylight and purple hues when viewed under incandescent light; Image: Bonhams
A red star spinel gemstone from Myanmar; Image: Bonhams

In the next instalment, learn about early sources for spinel gemstones and their history.



© Yaĝé Enigmus


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