About Spinel: Gemstone Information

Spinel is MgAl2O4 and commonly forms octahedra in the cubic crystal system though some specimens may show dodecahedron or cube faces. Twinning on {111} is frequent and spinel-law, penetration- or contactgroup types have been recorded. This direction sometimes shows parting but there is no cleavage. The hardness is 7.5–8, and the SG is 3.5–4.1 increasing with the iron or zinc content. Spinel may be transparent to nearly opaque and colours include brown, red, black, orange, yellow, green, blue, indigo and violet. Colourless spinel, at least of any size, will almost certainly be artificial. The single RI is 1.718. Spinel is formed at high temperatures as an accessory in igneous rocks, in regionally and contact metamorphosed limestones and in other contexts. Spinel forms three series, with magnesiochromite, with hercynite and with gahnite, the latter itself providing the occasional dark blue zinc-rich gemstone.
Very occasionally spinel may show an optical phenomenon, including a four- or six-rayed star, but the effect is adventitious. Red spinel is by far the most valuable, and as the finest red crystals occur with ruby in Myanmar, confusion can easily arise. With the spectroscope red spinel shows the chromium absorption spectrum but the bands in the blue, familiar in the ruby spectrum, are absent. In addition the emission doublet in the red spinel is replaced by several (up to five) emission lines, none of them forming a doublet. The strongest, at 686 nm and separated from the next strongest at 675 nm by a dark interval, is best seen with strong illumination and through a blue filter. Some Verneuil flame-fusion red spinels have been found to show a single emission line at 686 nm. The only other feature of the red spinel absorption spectrum is a wide band with its centre at 540 nm (the broad absorption band in ruby is centred at 550 nm). Blue spinel can cause trouble for students as the absorption
spectrum appears unexpectedly complex (this may be a purely subjective impression). Almost all blue spinels are a rather dark blue yet not really reminiscent of blue sapphire – some iolite may appear a similar colour but that stone of course has its startling pleochroism to distinguish it. The ferrous iron spectrum shows bands in the orange to green, at 635, 585, 555 and 508 nm; there is a narrow band in the blue at 478 nm and a stronger one at 458 nm; there are other lines at 443 and 433 nm. Red and pink spinels will show a crimson fluorescence under LWUV and a weaker response of similar colour under SW. Under X-rays there is a moderate glow of similar colour (but other less expensive testing methods should serve to identify spinel). A reddish response to LWUV has been noted in some purple to mauve specimens which show little reaction to SW but glow plum-colour to lilac under X-rays. Other specimens have been found to respond with a orange to red glow under LW, little response to SW and green under X-rays. Some pale mauve spinels may glow greenish under LW, a response also shown by some pale blue or violet blue stones.
Common inclusions in spinel are octahedra of other spinel group members, for example magnetite. Apatite crystals have been identified in red spinel both from Myanmar and Sri Lanka. In spinels from a zircon-rich, silica-poor context crystals of the zirconium oxide baddeleyite occur: Koivula and Gübelin (1992) illustrate examples and also show inclusions of dolomite, calcite and phlogopite (mica group) in Myanmar stones. Chains of three-phase inclusions are characteristic of spinel from Sri Lanka. Also characteristic of spinel are parallel rows of minute octahedra with either a white or a black filling. Quartz crystals have been found in Myanmar spinel. Koivula and Gübelin show a photograph of titanate crystals in a blue spinel from Sri Lanka; they also show an unhealed (dry) fissure in a spinel from the Hunza valley of Pakistan.
Spinel is a versatile species and may display 6-rayed stars and sometimes a colour change (dark blue in daylight to purplish blue in incandescent light). One such specimen showed an absorption spectrum with bands at 640, 580 and 540 nm in addition to the usual ones associated with ferrous iron. These bands are more often seen in the cobaltian synthetic spinel.
Until a few years ago any blue spinel showing red through the Chelsea filter would be considered a flame-fusion synthetic but some fine blue natural cobaltian blue spinels have been found in Sri Lanka. They show a very bright red (for a natural stone) through the filter and the blue is more attractive than that of the blue synthetic version. The first report on this material was made by Shigley and Stockton in 1984; they concluded that the presence of an absorption band at 460 nm was an indication that a specimen was natural whether or not cobalt was found to be present.
A report by GIA in 1986 further investigated cobalt-coloured blue spinel, finding that in the two specimens examined the RI was 1.720 and absorption bands showed at 622, 595, 575, 559, 489, 480 and 434 nm. Bands at 480, 460 and 434 nm were not found in synthetic material. GIA have also reported a natural twin blue star spinel of 2.12 ct; the star was doubled through twinning.
The finest red spinel comes from Myanmar and is found in association with ruby; attractive red and more rarely orange crystals are found in the gem gravels of Sri Lanka where the only reported cobaltian spinels occur. From Sri Lanka, too, comes the spinel at one time described as gahnospinel, with a composition intermediate between spinel and gahnite, the zinc spinel. This name is unnecessary; though interesting work was carried out by Anderson and Payne on the absorption spectra of the blue spinels (Anderson and Payne, 1998), they concluded that the visible absorption spectrum did not indicate the presence or absence of zinc. It is interesting to note that when ruby and spinel crystals are present together in gem gravels the spinel crystals frequently appear to have kept their shape better than the rubies; the reverse seems to be the case with blue sapphire and blue spinel.
Red spinel of gem quality has been found in Pakistan, the main locations in the Hunza Valley producing crystals of fine colour and generally larger than the Myanmar examples (Kazmi and O’Donoghue, 1990). The spinel is found with corundum in marble forming massive intercalations within garnetiferous mica schists and biotite-plagioclase gneisses. Characteristic of Pakistan spinels is a plum red-to-brown colour which I have not encountered from other locations. Characteristic inclusions in the Pakistan spinel are long prismatic crystals of a green amphibole and fine crystals of rutile. Nigeria, whose gem species are long overdue for a general description, has produced a fine blue spinel-type mineral from pegmatites in the Jemaa area: this has been identified as gahnite. The RI is in the range 1.793–1.794 and the SG in the range 4.4–4.9. This material has been found to glow red under incandescent lighting and shows two sharply defined areas with no absorption between them. Within the areas absorption is mixed in intensity. The defined areas are at 580 and 555 nm. Absorptions are also found in the red-yellow area, in similar locations to those found in other blue spinels and gahnites. Specimens heated in an oxidizing environment to 1000 °C for one hour showed a change from blue to blue-green; other stones heated to
1400 °C changed from blue to dark olive-green. A columbite group mineral has been found in the Nigerian spinels, with beryl and hematite. Deposits of red spinel from Badakhshan, Afghanistan, are of particular importance since one of them provided, among other celebrated stones, the Timur Ruby, now in the private collection of her Majesty Queen Elizabeth II. It forms the central stone of a necklace and weighs 352.54 ct. Bowersox
and Chamberlain, Gemstones of Afghanistan 1995; ISBN 0945005199, deals at some length with red spinel from these deposits and illustrates some of them. In addition, sale catalogues of Indian jewellery often give useful illustrations and captions – one example is Christie’s Important Indian jewellery sale held in London on 8 October 1997, in which the Carew spinel is illustrated and described. The spinel is inscribed following a Timurid fashion (the Timurid dynasty was founded by Tamerlane (Timur) and lasted from the late fourteenth to the late fifteenth centuries, ruling over Afghanistan, parts of Iran and central Asia).
The Kuh-i-Lal spinel deposit is 47 km south of Khorog, Tadjikistan, across the Amu Darya (Oxus river) from Shighnan, Afghanistan, on the edge of the Pyandzeh river valley (Bowersox and Chamberlain, 1995). These spinel mines are said to have been worked since about AD 950. In 1947 Fersman claimed that the white marble mines at the mouth of the Kuga-Lal river in the Pamirs had been producing red stones for 1000 years – bright rubies and pink to red spinels to which the name lal was given (Jewels of the Russian Diamond Fund, Gems & Gemology, 5 (11) 1947).
Other sites for the ancient mines have been suggested and it is possible that ruby and red spinel were confused with one another, as well as the names of the sites where they were found. Bowersox and Chamberlain made their own expedition to the area in 1994. They interviewed some of the inhabitants of the area and concluded that the site of the Kuh-i-Lal mine was across the Amu Darya from Shighnan; the authors give the coordinates. At the time of their visit military operations prevented a closer approach. Bowersox and Chamberlain quote from several authorities who give accounts of their own findings or speculations. It is possible that spinels may be found on both sides of the Ab-i-Pandja (Amu Darya), the sides today lying in both Tadjikistan and Afghanistan. The topography of the area has probably been affected by earth movements and landslides.
Spinel is not a particularly well-known species despite the fine red examples found in Mughal jewellery and (in England) the Black Prince’s ‘ruby’ found in the Crown Jewels. Spinel has often been referred to as ‘balas ruby’ and although the name should not be used in the scientific context it is often found in books on jewellery. In Crown Jewels of Iran (1968) (ISBN 802015190) Meen and Tushingham state that the largest red gem spinel known is a 500 ct specimen in the collection. Like most red gem spinels from Afghanistan the stone is not faceted but roughly polished, faceting not being found in general before the end of the eighteenth century. It is accompanied in the Iranian crown jewels by other red spinels similarly fashioned. Meen and Tushingham illustrate these stones in colour.