Duncan Miller

This is the first of an intended series of articles on faceting and polishing a variety of gemstones. I am beginning with quartz because that is what most people start faceting when they first take up the hobby. Quartz rough is inexpensive and readily available in a wide range of colours. It is not necessarily the easiest material to polish, but if a particular stone behaves badly it is no great loss to set it aside to be tackled at a later date. You should try to select rough that is free from cracks and veils. Smoky quartz rough should not be so dark that placed on white paper it looks black. Clean but slightly milky rose quartz can be facetted, usually with a pleasingly sleepy look. Colour in amethyst and citrine often is patchy or banded. In this case, orientate the stone so that any strong banding lies parallel to the table facet. A strongly coloured patch in an otherwise light stone should be positioned in the centre of the stone, although many references recommend you put it near the culet. A multitude of different minerals can be found as inclusions in quartz, and some of these can make very interesting gemstones.

If you are going to cut large quartz gems, it makes sense to remove unwanted bulk on a cabbing grindstone or a coarse lap, around 100 mesh grit size. This will leave considerable damage that needs to be removed with subsequent grinding steps. Depending on the size of the stone you need to work your way up the scale, coarse faceting with 325 or 600 mesh, and fine faceting with 1200 or 3000 mesh. I used to go straight to polish from 1200 mesh, but now I pre-polish with 3000 or 8000 mesh diamond paste on copper. This makes polishing quicker and easier. Quartz polishes best with cerium oxide or zirconium oxide. I haven’t found much difference between them. There are numerous lap alternatives for polishing quartz: UltraLaps; Corian; Lucite (Perspex); Darkside or Creamway from Gearloose Lapidaries; phenolic; old CDs, etc. I used to use a scored Lucite lap with cerium oxide mixed to a thin slurry but now I use Gearloose’s Creamway lap with the zirconium oxide Battstik crayon.

Deeply coloured quartz – amethyst, citrine, smoky – is pleochroic, so you may want to try to orientate your stone not only for colour banding and spots but also for the most desirable hue. Some intensely coloured amethyst, if orientated to take advantage of the blue-purple, can look almost like tanzanite. Bicoloured quartz, like ametrine, needs to be orientated in a suitably designed cut to allow separation of the colours, unless you deliberately want to mix the colours. Quartz has no strong cleavage, so apart from colour orientation you usually don’t need to worry about the crystallography. Prolonged polishing sometimes produces geometric relief on facet surfaces, particularly with amethyst. It is most noticeable in oblique lighting. This is due to Brazil law twinning, with different twin elements having slightly different polishing hardness. The way to minimise this is to have a very good prepolish, preferably with 8000 mesh, so that final polishing happens quickly.

Quartz is not particularly heat sensitive, so dopping with wax is my preference. Any hot stone can crack if the temperature changes suddenly, so avoid heating or cooling the stone too quickly. Polishing on some laps, like Lucite and phenolic, can heat the stone to the point of softening the wax and allowing the stone to shift on the dop. Be aware of this and keep the stone cool with damp paper towelling if necessary. Quartz is quite brittle, and pavilion keel facets need to be polished with the lap running parallel to the keel edge to avoid chipping. Apart from the problem of polishing out chips, you don’t want a quartz chip to embed in your polishing lap. The residue from grinding can also foul faceting laps, so don’t let your laps dry out with quartz powder on them. It will set hard and be difficult to remove. Wash each lap thoroughly immediately after use. If you following these guidelines you should have no difficulty faceting and polishing quartz.



                       A.                                                             B.                                                                        C.

A - 110 ct natural citrine. Tripolar Brilliant, cut by Duncan Miller

     B - Typical damage in quartz caused by coarse diamond grinding

C - Geometric polishing relief due to Brazil law twinning in quartz