Articles

Duncan Miller

Inclusions in gemstones often are seen as just a nuisance by faceters, who find themselves urged to buy only ‘clean’ rough. I suppose it is a matter of taste, but inclusions that do not detract from the visual appearance of a gemstone can aid in proving its authenticity. And some inclusions definitely enhance the value and appearance of certain gems. A visible ‘horse tail’ inclusion of asbestos fibres in Russian demantoid is perhaps the most famous example of desirable inclusions, not that many people will get to see a real life example. But rutilated quartz would just be plain quartz without the rutile needles, and a multitude of different minerals can make attractive inclusion scenery in quartz. Some inclusions are so characteristic of their host material, that examples without them are rare. Lily-pad inclusions, small disc-like cracks around minute black inclusions in peridot, are a good example.

Undesirable inclusions that cannot be removed by pre-forming often can be hidden partially beneath crown main and break facets. Avoid having these placed in the centre of a stone, where they will be multiplied by reflection in the pavilion facets. The very worst place to allow an undesirable inclusion to lurk is in the culet, so remove any damaged or cracked ‘skin’ from your rough before blocking out the pavilion main facets. Fine tubular inclusions, often seen in aquamarine, can be orientated vertically in the stone to minimise their visual intrusion. Stones containing a multitude of finely dispersed inclusions may appear ‘sleepy’ or milky, like much rose quartz. Such stones can be cut into very beautiful gems with some crown facets left frosted to increase the contrast between them and polished surfaces.

Orientating to display desirable inclusions involves the inverse of hiding them. For example, many cutters search for years for that piece of quartz or topaz with a single needle of rutile or tourmaline to orientate vertically through the centre of the stone. This will produce multiple reflections if centred properly, but how do you get it dead centre? I am sure there are other possibly more elegant solutions to the problem but I roughly preformed the stone, used a drill with a ball burr to grind two small hollows in the opposite ends of the rutile needle, and clamped the stone between two pointed dops in the transfer block before fixing the stone to one dop firmly with lots of wax. Then I removed the other dop, replaced it with a face-plate dop, and proceeded as if doing a transfer. The result was a needle perfectly centred in line with the dop axis to cut a round brilliant. (Unfortunately in this instance the rutile needle was very fine, which compromise the result. Perhaps one should just drill a hole through a stone and fill it with dye . . .)

Orientating other desirable inclusions depends on their nature and the shape of the rough. Deliberate central placement will cause multiple reflections. Perhaps you want two dissimilar inclusions side by side. It often is a good idea to avoid having inclusions breaking the surface of the crown. The difference in hardness relative to the host could leave polishing hollows or raised bumps. Once, when cutting large facets on some rutilated quartz, I was frustrated by some of the straight needles actually pushing through the quartz and emerging slightly on the other side. This would have caused a polishing disaster. So, what to do? A drop of cyano-acrylate glue on either end tethered the needles so that the little projections could be removed in pre-polishing to avoid scoring the polishing lap.

So next time you are selecting rough, don’t automatically discard included stones. Think creatively what you might be able to do with them to enhance their appearance and increase their value by cleverly including the inclusions. All the gems in the illustrations were cut by Duncan Miller.

Centreing a rutile needle in the transfer fixture between two dops ground to points

A 17,77 ct scapolite from Merelani tanzanite mines, with unidentified inclusions like ink spots>/p>