Faceting and Polishing Quartz

January 15, 2017

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

 

Playing With Stars

November 23, 2016

I cut off the end of a damaged Goboboseb quartz crystal because it had a deep purple central inclusion at the one end, which I thought would make an interesting stone to facet.

 But then I noticed that the end of the remaining piece had regular purple stripes radiating from the centre to the points of the hexagonal crystal. So I cut off another section of the crystal to the depth I hoped I would need for cutting a gem.

I first tried to find the middle of the purple star shape, and marked s...


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Who Knows Nigel?

October 23, 2016

Not many people in our club know Nigel Brown, but behind the scenes he must be one of our most productive lapidaries, in and among all his other commitments. His website has been listed in this newsletter’s advertisements for several months now (nigelbrownjadecarver.com). When I looked at it again recently I saw he had been busy producing jade kiwis so I asked him for an update of his work. He sent us this: 

Little Spotted Kiwi

This little...


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THE C-AXIS, WHAT IT IS AND WHY IT IS USEFUL TO GEM CUTTERS

September 23, 2016
by Duncan Miller

All crystals fall into one of seven crystal systems, based on their symmetry. In crystal drawings, by convention, the c-axis usually is orientated vertically, in the plane of the paper. All crystals except those in the cubic (or isometric) crystal system have a c-axis. Cubic system crystals, like diamond, garnet and spinel, have no c-axis because all three crystallographic axes are necessarily the same length. In the other crystal systems the c-axis can be longer or shorter th...


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THE SECRET OF A GOOD POLISH – IS A GOOD PRE-POLISH

August 28, 2016

In the bad old days, one cut facets on a 600 mesh lap, followed by a 1200 mesh lap and then went on to polish. The 1200 mesh leaves quite deep scratches, and on some material produces ‘orange peel’, a mottled surface with alternating rough and smooth patches. This makes polishing tedious. A pre-polishing step, with 3000 mesh or 8000 mesh diamond gets rid of the scratches and any orange peel. You might think the additional step adds time to the process, but in practice it speeds it up beca...


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Selecting Rough

June 25, 2016

By Duncan Miller

If you are going to facet, you need to learn something about mineralogy because you need to know what stones you should obtain, how their characteristics affect their behaviour while you are cutting and polishing them, and how they affect the optical properties of your finished gemstone. The easiest material for beginners to cut and polish is common red garnet. It presents no problem with cleavage or orientation for colour, and generally behaves itself well during ‘cutting...


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Skorpion Mine, Rosh Pinah, Namibia - some rare and recent minerals

June 25, 2016

By Gisela Hinder, Rosh Pinah Geo Centre


The Skorpion non-sulphide zinc mine in southwestern Namibia has always produced interesting and rare minerals. To name only a few amongst the great variety of zinc carbonates, phosphates and silicates discovered at Skorpion, the skorpionite, hemimorphite, smithsonite and tarbuttite crystals are probably the best known. Nevertheless, Skorpion mine has an area where copper values in the host rock are higher, and minerals like malachite, chrysocolla, zincol...

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Azurite Suns: Mineral Masterpieces from Australia

May 25, 2016

Reproduced with the permission of Eric Greene of Treasure Mountain Mining

Azurite is a soft, deep blue copper mineral produced by the oxidation weathering of copper ore deposits. It is a favourite amongst mineral collectors because of its rich blue colour and wide availability in a variety of forms and colour variations, from sharp, lustrous brilliant dark blue crystals to thick, rich, colourful royal blue coatings on matrix.

Azurite suns are a unique form of azurite that has been found in ...


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Re-polishing a Table Facet

May 4, 2016
Anyone who re-polishes worn stones, or who tries to remove a scratch from a table facet, will be familiar with a common problem. Some stones, in my experience particularly tourmaline, appear to develop a resistant ‘skin’ during polishing, which impedes the re-polishing process. The effect is that you cannot re-polish the facet, which just slides over the lap, with your usual polishing combination. I think it is due to work-hardening of a surface layer; but there are other opinions about w...
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Water Splash Covers

March 24, 2016
To reduce water spray when facet cutting at high speed, use a splash guard cut from the lid of a cheap plastic bucket, or alternatively use a trimmed-down cake fruit mix bucket when cutting girdles on a Raytech faceting machine.


  
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