HOW TO CUT TOURMALINE
Tourmaline is to me a truly fascinating gemstone. I am attracted by the wide range of colours in which it occurs and by its property of dichroism which can lead to even greater variety within the same stone. Don't worry about the term dichroism if this is new to you. Any gem crystal that is dichroic has a structure which absorbs light differently depending on which way it passes through. This property is only
significant optically from a cutters point of view. A piece of tourmaline without obvious physical problems will cut and polish well in any direction, but looking at how the light is behaving gives us some choices as to how the finished stone will appear.
Tourmaline is sometimes tumble polished, but it is more usual to cut cabochons and faceted stones from the better qualities available. It is difficult to do justice to the range of colours in words, but they are usually rich and often tend to darker rather athan lighter shades. A crystal may be uniformly coloured or may show a range of shades of the same colour or two or more different colours.
At this point it is worth looking at the shape of a typical crystal. Its long axis, or c-axis is usually easy to discover as the outer faces of a crystal often have lines or striations running in the same direction. This means that even with a small or oddly-shaped piece displaying a small area of the outside it can be viewed with light passing through along the c-axis.
A typical crystal 1c-axis
Back to colour. Looking along the c-axis it will almost always be darker. Sometimes this can be very dark, tending to dark brown or black for some of the green or blue varieties. Often it is a darker shade of the same colour seen when looking at the crystal across the c-axis. This is the effect of dichroism. Crystals with more than one body colour can exhibit this property in both directions described. Slices are often cut
across the crystal which show the concentric bands of colour to best effect. When red or pink in the centre with a green surround this is described as 'Watermelon' tourmaline, but a whole range of colour combinations are possible and frequently found. In the other direction the same applies, but stones cut this way may display two or more distinct colours with a precise plane at which the colour changes. Green
and pink stones of good quality can be found, but care must be taken in cutting as sometimes the boundary may also be a physical weakness.
It is possible to choose how a cabochon cut stone will appear by sawing or grinding its base square to a favourite viewing direction. A better colour may be obtained from dark stones by cutting lower domes. Polishing the base of the cabochon is essential for the best possible appearance. Other than taking care to avoid areas which have fractures or are heavily included, no special treatment is necessary
compared with other stones. Tourmaline cuts and polishes easily with usual lapidary techniques and materials. Inclusions intourmaline can sometimes be found in considerable quantities. This is not necessarily bad news. Long fine tubes, parallell with the c-axis will sometimes produce good quality cats-eye stones especially if the tubes are fine and the body colour of the stone is attractive. As with all cats-eye
material, the base of the cabochon must be parallel to the line of the inclusions with even doming to get a good 'eye'. Some liquid-filled inclusions can give an attractive play of colour in paler stones, but I have only found a few of these.
When looking at a piece of material for a faceted stone the same considerations apply, but a little more care is needed. The beauty of a faceted stone depends not only on its colour, but on the light reflected back to the viewer from the facets on the underside, or pavillion. With the top face or table facet of the stone square to the c-axis the colour willbe uniform if the stone itself shows a uniform body
colour, without separate zones. With lighter shades this is probably the best orientation to aim for, but as we have already considered this may not be the most attractive colour. With the table facet parallel to the c-axis tourmaline crystals often show their best colour and their shape often leads to economical cutting in this direction. However, some facets will reflect different colours as light is reflected back
from within the stone in various directions. This is desirable where the colours are complimentary, but may be less so if darker brown tones are introduced. If this is the case it is good practice to design and orientate the stone so that these facets are kept to the ends and minimise the reflections by keeping the angles of these steep. Tourmaline is most frequently found in rectangular or long emerald cut shapes for these reasons.
If you are buying stones for jewellery, the way the cutter has taken account of these properties will influence the attractiveness of the stone.
If you are buying crystals or broken pieces for cutting, a little time spent viewing them in different directions is worthwhile.
If you are like me and are curious about stones you will find tourmaline offers the possibility of something new with each piece