Spinel is a gemstone found in greater abundance in Sri Lanka than either corundum or chrysoberyl. Its very abundance makes Sri Lanka the second largest producer of this stone next to Myanmar (Burma). Spinel is found in association with other varieties of gemstones among the gem gravels of this country. These are more closely associated with corundum, being found in the same environment and being deposited in much the same manner. These also show similar colour resemblances to the corundum varieties particularly in respect of red, orangish red, blue, mauve and the colourless. Nevertheless these arc two distinctly different mineral groups.
Chemically, spinel is an oxide of aluminium and magnesium and belongs to an isomorphous group, where a certain amount of ferrous, manganous or zinc oxide could replace some of its magnesia, while ferric or chromic oxide could replace some of the alumina. Because of this isomorphous replacement its composition varies slightly. Consequent to this variation in its chemistry, slight differences in their optical and physical properties are seen in the different varieties. Spinel crystallizes in the cubic system and its most common crystal habit is octahedral. These have a specific gravity that varies between 3.60 and 3.63 and a refractive index which varies between 1.710 and 1.717 for the gem varieties. Apart from the common varieties of spinel are also varieties identified as Ceylonite, Gahnite and Ghanospinel. These varieties because of their differences in composition which is the result of isomorphous replacement have much higher readings in their optical and physical 'constants' such as specific gravity and refractive index.
Ceylonite is a magnesium iron aluminium oxide (Mg1 Fe, Al2 O4). This variety has been quite rightly named after Sri Lanka (Ceylon) in view of its reputation for this variety of spinel. It is an iron rich variety which is of a very dark green, grading to dark greenish-black or black. The black varieties are opaque and are of very little significance as gemstones. Nevertheless these could take a very good polish and are then attractive in their own way and could be used in modern jewellery settings. Ceylonite is found as waterworn pebbles among the gem gravels of this country. These have a refractive index that varies between 1.77 and 1.78 and a density within the range of 3.63 and 3.90.
Gahnite is a zinc aluminium oxide and rich in zinc. These stones are green in colour and transparent materials when facetted are attractive. This material has been so named after its discoverer G.J. Gahn a Swedish chemist These have very high densities recording a range between 4.0 and 4.62 and a refractive index around 1.805. However the Nigerian varieties as recorded by Jackson (1982) are said to have a refractive index ranging between 1.793 and 1.794 and a density varying between 4.400 and 4.589. This could be attributed to the varying percentages of zinc in the composition.
Ghanospinel is a magnesium zinc aluminium oxide occurring normally in very dark colours - dark blue and bluish black. It could be said that this variety is invariably found among the gem gravels of Sri Lanka. It has a density that varies within a wide range between 3.58 and 4.06. Its refractive index also varies between 1.725 and 1.753. An interesting revelation has been made through the researches of Anderson (1964). Accordingly, certain light blue spinels which apparently gave no indication of excessive iron, and displayed a normal appearance were found to have very high densities and refractive indices. The crux of the revelation was that the presence of zinc was the cause. Most of the test samples were blue spinels from Sri Lanka. The conclusion arrived at thereby was that certain blue spinels of pale colour and normal appearance from Sri Lanka could have surprisingly high densities and optics suggesting the presence of zinc. Further reference has been made to this by Anderson (1974). A reference has also been made to a ghanospinel octahedron crystal from Ratnapura weighing 4.3 grams (Frenzel etal 1986).
Recent research by Schmetzer and Bank (1986) shows that colour absorption spectra, and inclusions in zinc-bearing ghanospinel were found to be similar to the properties of ordinary zinc free spinel from Sri Lanka.
The colours in which spinel occurs are red, pink, orange, shades of reddish-purple, blue, bluish-green, mauve and colourless. Ceylonite is usually greenish black to black in colour and is almost opaque. Spinel occurs in varying degrees of transparency ranging from transparent to opaque. The cause of colour in spinel is the colouring ions chromium in the reds, ferrous iron or cobalt in the blues and allied shades, and manganese in the purple stones. The occurrence of natural blue spinel coloured by cobalt has been found in Sri Lanka. This refutes the original theory that only synthetic stones had cobalt as their colouring agent and that the presence of cobalt was conclusive enough to declare such stones as synthetic productions. However, beautiful natural blue spinel coloured with cobalt are now available. Harder (1986) has recorded that cobalt bearing spinel having traces of iron, nickel, vanadium and gallium has been found in the gem gravels around Paradise Estate mining area in the vicinity of Ratnapura. Cobalt spinel has also been found around Okkampitiya and Embilipitiya.
Spinel has a hardness of 8, occupying the fourth place in Mohs's scale of hardness. Being hard, spinel is able to lend itself to a high degree of polish. The red and reddish purple stones show colour resemblances to rubies and garnets but are easily differentiated. Large, fine gem quality spinel exceeding 10 carats are rare. Blue spinel is somewhat more abundant, and beautiful dark blue stones of a good lustre are not uncommon. Of the more common colours are blues slightly inclined to purple, mauve, brownish-red and greenish-blue stones, while colourless stones do occur but are rare.
Asteriated spinel with either four or six rays are also found in the gravels of Sri Lanka. Generally a majority of asteriated spinel are of bluish or brownish-red and purplish-red colours. The cause of asterism is the same as in the case of corundum, being a reflection effect off inclusions. Investigations of Bank (1980) have revealed that the inclusions causing this star effect are rutile needles. He also states that for the first time star spinel from Sri Lanka were obtained in 1979.
Synthetic spinels that compare very closely to their natural counterparts are being manufactured. However, their optical behavior and their physical characteristics differ and this makes detection easy.
Colour changing 'alexandrite-like' spinel have also been found from time to time in this country. Here the original colours are quite different to alexandrite, very often being violet in daylight and changing to reddish violet in incandescent light. An analysis of such material by Schmetzer and Gubelin (1980) has revealed the composition to be iron, chromium and vanadium. Colour changing blue spinels have also been found recently in this country among its gem gravels. The colour changes to a purplish shade under incandescent light.
A deep red to black manganese aluminum oxide variety identified as galaxite is not known to occur in Sri Lanka but mentioned here as information. These have a very high refractive index around 1.92 and a density of around 4.05.