• Gem 2-1
  • Gem 2-2
  • Gem 2-3

TourmalineThe term Tourmaline has been apparently derived from the Sinhalese term “Thoramalli”. As for the origin of the term “Thoramalli” itself, a very plausible explanation among the local people is that it bears the identical colour resemblance to the flower of a common wild plant, identified as “thora”. The flowers of these plants are yellow and has a brown calyx which covers about one third of the flower giving an overall yellowish – brown appearance. It could be said that this colour matches perfectly the colour of s yellowish – brown tourmaline, The yellowish brown tourmaline are more abundant in Sri Lanka compared to other colour varieties. As Sri Lanka has a reputation for the brownish yellow tourmaline it is possible that the term tourmaline is a combination of the Sinhalese term ‘ thora.’, the name of the plant and ‘mal’ the Sinhalese term for flower (thora-mal).

In Sri Lanka the term tourmaline is rather indiscriminately used to embrace certain varieties of gemstones which have similar appearances. It is not uncommon to find zircon, andalusite, sinhalite, epidote, enstatite, konerupine and such like minerals in a parcel of rough gemstones offered for sale, as tourmaline. This is so even in the case of polished material. Now however dealers are more conversant and knowledgeable in this respect and instances of misidentification are less frequent.

Tourmalines are an ismorphous group of minerals and their chemical composition is very complex and variable. It is so complex that it is difficult to describe it under a general formula. It is a silicate of born and aluminium and there are always present in all varieties of tourmaline other elements like magnesium, sodium, potassium and water. Flurine, Lithium and manganese enter into the composition of a few while the presence of the amount of iron is variable though important. Analyses have revaled that the silica- boron ratio is fairly constant while great variations are seen in the ratios of other constituents. These variations in the composition cause differences in colour, degrees of transparency and slight differences in the optical and physical properties like, refractive indices, density and hardness. The refractive indices within the tourmaline group could vary between 1.62 and 1.65, the density between 3.00 and 3.2 and its hardness could vary between 7 and 7.5. It is interesting to note that inspite of such chemical, optical and physical diversity which gives rise to different varieties of a range of different colours, tourmaline as a group crystallizes in the trigonal system. The crystals are characterized by vertical striations along their prism lengths. Corresponding to the complexities of composition the varietal classification of tourmaline is a very complex one. The classification becomes even more complex when one observes a further classification of tourmaline in relation to colours in which they occur, and which also corresponds closely to the composition. On the basis of the composition of tourmaline and the colours these display, distinct varieties are identified, in relation to both, for scientific and trade purposes, as follows : Dravite, Uvaite, Liddicolite, Elbaite, Rubellite, Siberite, Indicolite, Tsilaisite, Schorl and Achroite. The various colours in which tourmaline occur are due to the basic and essential ingredients in their composition. Tourmalines are found in varying degrees of transparency and these are highly dichroic.Dichroism is most apparent in dark colours which could even completely impair the passage of light in certain directions. Among tourmalines are quite a range of colour varieties. Those of good and pleasing shades of colour, sufficiently transparent and flawless are fashioned as gemstones. In respect of colour it is seen that certain crystals are parti- coloured having more than one colour in them, different portion being of different colours. The different colours could be seen along the length of the crystal and here the colours are at the two ends where the colour demarcations are very abrupt and sharp. Sometimes colour zoning is seen in a concentric arrangement. This is most apparent if the crystal is sliced in a direction perpendicular to the vertical axis. Then it will be seen that the core is differently coloured to the periphery and sometimes there is a lighter shade of colour in between as well. Mitchell (1984) describes this feature thus: “crystals can vary in colour concentrically around the c- axis or even vary in colour along the length of the crystal.” Those tourmalines with concentric zoning of colour are referred to as ‘watermelon’ tourmaline. Very often the colours are red and green. Michel (1984) however continues further by making reference to his own collection of parti- coloured tourmaline sighting samples that have the colours, yellow and green, gray and red and blue and pale yellow. He also makes an interesting reference to a specimen in his collection which is red and green chatoyant tourmaline, which indeed is a rare occurrence. Material with concentric zoning of colours has as yet not been recorded from Sri Lanka. The numerous colours of tourmalines are due to the basic and essential ingredients in their composition. Analyses have revealed that the rose- red, dark red and light green colours are due to the presence of small amounts of manganese and lithium. The red and reddish varieties are identified as rubellite and in instances where the shade of red carries a purplish tint, these are identified as siberite. These varieties too are not known to occur in Sri Lanka, The light green colours are caused by ferrous oxide and in certain cases these have been found to contain faint traces of chromic oxide in which instance the green, in much brighter. The Lankan green products have always been of a dull green, the colours being dampened by the presence of ferrous oxide, these are more yellowish-green in appearance, and Apart from these colours are the brown, brownish yellow and the honey yellow colours which are really the calcium magnesium and sodium magnesium varieties of the family. These varieties are broadly identified as uvaite and dravite respectively, Most of the brown and yellowish brown varieties arc located mainly around Uva and Ratnapura and Tissamaharama regions. Uvaite has been named after the province of Uva where these varieties are most abundant and widespread. The other areas in which tourmalines are found widespread and in fair abundance are the Lunugala, Bibile, Passara, Nilgala regions, the Horana, Matugama, Pelawatte, Marawaka, Deniyaya, Rakwana areas, around Ratnapura, Avissawella and Haputale, and also around Ambalantota in the south.

Tourmalines that are of a shade of blue are termed indicolite. These could be either dark or light in colour or could be of an indigo blue colour which apparently has given it its name. These very often reveal a tinge of green when closely observed. Deep indigo blue varieties of good transparency are highly prized, for such colours with perfect transparency are rare; the lighter shades being the more common. In Sri Lanka indicolite is a rare commodity. Here again ferrous oxide is responsible for the various shades.

The brown and the green varieties are fairly common in the country. As indicated these are always tinged with yellow. The colours are due mainly to ferrous oxide while lithium and manganese could be contributory.

Chatoyant tourmalines are by no means uncommon and those displaying beautiful cat's eye effects are met with. As has been mentioned earlier chatoyancy in gem minerals is a reflection effect caused by acicular mineral inclusions or microscopic hollows or tubes which are assembled in a particular orientation to the crystal structure of the host mineral. A study of chatoyancy in tourmaline made by Graziani et. al (1982) has revealed that the effect is caused by acicular mineral inclusions or microscopic tubes. They expressed the opinion that the needle like inclusions were none other than tourmaline in most cases, except for a few which contained epidote. This revelation has been made after micro chemical analyses.

Colour changing chromiferrous tourmaline from East Africa has been recorded by Bank and Henn (1988). These green tourmalines are said to be green in daylight, changing to brownish red in artificial incandescent light. A chemical analysis has revealed these to be chromiferrous with a dravite - uvaite mixed composition and with a slight excess of calcium (uvaite content).