Tourmaline is a beautiful gemstone that forms in various colours, occasionally even as a layered rainbow of more than one shade that is termed bi-colour or tri-colour.
These colour variations can occur parallel to the c-axis and are known as watermelon tourmalines when cut. Examples where colours occur parallel to the a-axes are typically step-cut with the table parallel to the c-axis displaying zones of colours along the length of the faceted gemstone.
If numerous parallel tubes are regularly orientated within the gemstone then it may be cut ‘en cabochon’, displaying an attractive cat’s eye. Cat’s eye or star gemstones are often called phenomenal gemstones.
The most exciting, rare, expensive and highly-prized colour of tourmaline found in the 1980s was the exquisite turquoise colour called Paraiba from the Brazilian state of the same name. It is coloured by copper and manganese.
Since 2001, similar coloured tourmalines have been found in Nigeria and this is attributed to the continental drift, which occurred millions of years ago. At that time the east coast of South America is believed to have been attached to the west coast of Africa, thus Nigeria fitted closely by the north-east of Brazil and therefore shared the same geological occurrence.
The cause of colour in all tourmalines is not fully understood at this time and study continues in this field. With a hardness of 7 to 7.5 on Mohs Scale, they are durable and suitable for most jewellery, including rings.
Heat treatment to lighten dark blue or green tourmalines may be undertaken or irradiation may be used to intensify pink tourmalines. Both of these methods are stable.
Colourless tourmaline occurs in nature but pinkish colours can be heat-treated to make them colourless. Green tourmalines come in a variety of shades and sometimes the lighter colours are heated to enrich the colour.
Tourmalines are noted for inclusions called trichites, which present as thread-like or fine hair inclusions. Under fairly strong magnification they may resolve themselves as two-phase inclusions consisting of a liquid phase with a vapour bubble (Figure 1). An interesting feature in bi coloured or tri-coloured gemstones is that the trichites often cross colour-zone boundaries without disruption to their form.
Growth tubes are common and are usually parallel to the c-axis. They may be filled with liquids, minerals, or epigenetic matter that has entered the gemstone after its formation.
Cracks and fissures are common in crystals from most deposits and may be mistaken for cleavage. These cracks and fissures occur perpendicular to the c-axis.
Tourmaline may contain different mineral inclusions, including tourmaline itself, which are difficult to locate or see, as the refractive index is the same as its host. A polariscope fitted to the microscope will readily reveal them – cross the polaroid filters, place the gemstone in between them and rotate the polariscope gently and the included crystal will shine brightly with pretty colours (Figure 2).
The importance of tourmaline in the jewellery industry is that it comes in a variety of colours and is very durable.