TOURMALINE
April 25, 2017Duncan Miller
Tourmaline is pleochroic, sometimes strongly, so the rough must be orientated for colour. Blue and green crystals usually show the best colour transverse to the c-axis. Some are so dark in the direction of the c-axis that they are ‘closed’, and need to be cut with steep pavilion facets at the short ends, to minimise darkening of the finished gem. ‘Open c-axis’ green and blue tourmalines command a premium. Pink tourmalines, by contrast, often show the best colour viewed parallel to the c-axis and the table is best placed perpendicular to it, if the rough allows. Despite these constraints, well-shaped tourmaline rough can give you very good weight return after cutting. When choosing tourmaline rough, remember to do the white paper test and reject any rough that is too dark to allow you to see good colour through the crystal placed on white paper in strong indirect light. Avoid long, narrow crystals. They tend to break and long, narrow stones are difficult to sell or set.
Faceting tourmaline, once you have removed any peripheral cracks, is quite easy. I cut large facets on 600 mesh, then recut them and cut finer facets on 1200 mesh, followed by a 8000 mesh pre-polish and polish on a Gearloose Lapidary’s Matrix lap with aluminium oxide Battstik. Formerly I used to use tin/lean or a Batt lap with Linde A made up into a thin slurry, the consistency of milk. Less rather than more polishing agent is better, to avoid streaking or scratching. Unless your laps are new and very flat, long facets need to be cut and polished with the length parallel to the running direction, or the end up curved. Curved facets can cause problems when re-polishing a commercially cut stone, and often complete re-cutting is preferable.
Another problem when trying to re-polish a tourmaline, especially the table facet, is that you need to remove the original polished layer with a relatively coarse lap, at least 1200 mesh and often 600 mesh. Initially this produces a very rough surface, so usually this necessitates re-cutting and polishing the adjacent crown facets too. I think this ‘hard’ surface layer is due to plastic work-hardnening during polishing, but I have no way of proving this assertion. This phenomenon occurs on other stones as well, including sapphire, but doesn’t seem as pronounced as on tourmaline. Tourmaline also has quite marked differential hardness and end facets on long stones, transverse to the c-axis polish differently from those parallel to it. A light touch here and sweeping the lap avoids streaking and grooving of these facets.
Tourmalines come in every colour imaginable, and some mixes of colours are very captivating. But avoid stones that have green and pink pleochroism (although they make wonderful mineral specimens) because the blended colour is a muddy khaki, unless you plan an elongated stone to take advantage of the dichroism.
Top
quality 65,91 ct green tourmaline rough from Namibia . . .
. . . and the 21,90 ct gem I cut from it.
Posted by Site Moderator Webmaster. Posted In : Faceting
