THE POLARISCOPE, THE FACETER’S FRIEND

September 23, 2017

Duncan Miller

A polariscope consists essentially of two polaroid filters, or a source of plane polarised light and one polaroid filter. The source of polarised light can be a white computer screen or even the sky, viewed at 90 degrees to the Sun. For the filter, or analyser, you can use a sheet of polaroid, or a lens from a cheap pair of 3D movie spectacles.

Let’s start with a white computer flat screen. Even an older cell phone screen without a plastic cover produces plane polarised light. Rotate an elongated transparent tourmaline crystal in front of your white computer screen and see what happens. It goes from light to dark four times in a full rotation and the colour changes. You are seeing the pleochroic colours of the dichroic tourmaline crystal. Now take up the analysing polaroid filter or 3D movie glasses and rotate these in front of the white computer screen. What do you see? Four times in a full rotation the polarising filter goes black. (You may have to look through the ‘front’ of the 3D movie glasses to see this.) Now hold the analysing filter in the black position (called ‘crossed polars’) and rotate your elongated tourmaline crystal, or an elongated quarz or beryl crystal, between the filter and the computer screen. What do you see? The crystal goes dark four times in a full rotation. Try this with other minerals – a sheet of mica, a rhomb of calcite, etc. – and then with a piece of glass. The glass will stay dark for the full rotation, when most of the other minerals will blink from dark to light. If you experiment long enough you may find that some minerals that blink on and off in some directions stay dark in others. So we have some explaining to do.

Glass, plastic, and minerals in the cubic system – including diamond, spinel, garnet – are optically isotropic and should stay dark for rotation in any orientation between crossed polars. In reality they may not stay completely dark, but waver between dark and light as you rotate them, and if they are strained they may show waves of bright colours. But with some practice you will distinguish this behaviour from that of the crystals that are not isotropic – like tourmaline, quartz, beryl, etc. – that in most orientations blink on and off, light/dark four times in a full rotation between crossed polars. So there you have one means of possibly distinguishing between glass and some common gem minerals with your polariscope set-up.

Optically uniaxial minerals, like quartz, beryl and tourmaline, have one direction in which they do not blink light and dark between crossed polars. This is the direction of the c-axis, and often these crystal are elongated parallel to this direction. It can be useful to be able to determine this direction in rough, especially if it is an irregular lump of nicely coloured rough. In aquamarine and pink tourmaline, for example, this is the direction of best colour and ideally the table of the gem should be perpendicular to it.

Twinning can cause opposite bands of dark and light under crossed polars. This is how people looking for twinned calcite can determine quickly in advance if the rough is twinned, and the orientation of the twin planes. These need to be orientated at an oblique angle to the table facet in order to produce the rainbow interference colours displayed by some faceted calcite.

A more sophisticated polariscope consists of a stand, with a light in the base and two polaroid filters. The upper one can be rotated into the dark position. This allows you to manipulate the crystal or cut gemstone more easily, to gain more information. Faceted synthetic quartz often is cut with the table perpendicular to the c-axis. Under crossed polars with the c-axis orientated vertically, a facetted synthetic quartz often displays bright interference colours. If you insert a 10× lens between the stone and the analyser, with luck you may see a typical quartz ‘bulls-eye’ interference figure, of a dark cross with concentric coloured rings around a coloured or colourless centre. Other uniaxial minerals in similar orientation will produce a similar interference figure of the dark cross with concentric coloured rings, but without the bull’s eye centre. So this can help you distinguish quartz from other uniaxial minerals or glass. If you have a quartz sphere, or even a quartz bead, rotate it between crossed polars and see what happens. In one orientation it will produce an interference figure magically. Then you are looking straight down the c-axis. There are other more sophisticated things you can do with a polariscope, but for the faceter it is a quick and easy way to distinguish glass from common minerals like quartz and beryl, and is very useful for finding the c-axis in irregular uniaxial rough. It is easy to experiment with polarised light, without having to invest in expensive equipment, and you can learn a great deal about the practical applications of crystal optics in faceting and gemmology without having to get to grips with the complicated physics (see ‘How to play with polarized light’ and more detailed information at http://homepage.ruhr-uni-bochum.de/Olaf.Medenbach/eng.html)

 

A polariscope used with a magnifying lens to photograph the interference figure produced by a scapolite crystal.

 

The slightly distorted uniaxial interference figure of scapolite viewed down the c-axis in the polariscope with a 10× magnifying lens, as in the photograph above.

 

Synthetic quartz cut with the table perpendicular to the c-axis, showing bright interference colours viewed in the polariscope with crossed polars. Inserting a magnifying lens between the stone and the upper polarising filter would produce a typical bull’s eye interference figure for quartz.

 

 

Slice through a quartz crystal perpendicular to the c-axis, under crossed polars, showing dark wedges due to Brazil law twinning

 

 
Twinning in calcite revealed by interference colours under crossed polars in the polariscope

 

9th September, 2017. The Jan Coetzee Quartz Crystal Reunion

September 23, 2017

About ten intrepid Jan Coetzee crystals made it to the reunion, with two of 50 kg-plus guys sending their apologies due to being overweight, along with a couple more whose owner couldn’t provide transport on the day. One rare and seldom seen fluorite (also from the same crystal pocket) came, and was much admired. Malcolm Jackson gave a short presentation about the mine and its location, along with photos taken by him and Jo of the Jan Coetzee mine dumps in recent years and the remains of th...


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FACETING THE NAMIBIAN RARITIES

August 24, 2017

Duncan Miller

During the 1974/75 university holidays I was fortunate to work for Sid Pieters in Windhoek for several months. It was a wonderful experience, including seeing some of the most famous mineral specimens then coming out of Tsumeb, but also to encounter some very special gem materials. Through Sid Pieters’s generosity I returned home to Cape Town with a few small fragments of jeremejevite from the original Namibian occurrence at Cape Cross and some pieces of cuprite from Onganja t...


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Curling Stones

August 24, 2017

Lesley Andrews gave a most interesting talk on Scottish curling stones. I thought curling was a Scottish winter game played by village yokels. I was wrong! It has had Winter Olympic status since 1998. The game consists of two teams of four players each, with eight stones between them, and the idea is to slide the stone, which turns, hence the name curling, towards a target called a button. Rather like a game of bowls on ice. The origin of the game goes back into obscurity, but the oldest know...


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FACETING FOR INCLUSIONS

July 24, 2017

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 ...


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Synthetics

June 27, 2017

Duncan Miller

Synthetics are a wonderful source of relatively inexpensive faceting rough, in a wide array of colours, some of them not available at all in natural stones. On the whole, synthetic gem rough is predictable in its behaviour and also enables the cutter to explore quirky cuts in larger sizes than would be affordable in natural rough. And increasingly jewellers are setting well-cut synthetics in precious metal jewellery. So dive in, and enjoy yourself.

The most commonly available ...


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New Barite Occurrence at Rosh Pinah Mine – Namibia

May 23, 2017

Transparent to whitish barite mineralization was found in an orogenic late phase leached fault zone. It seems that some of the barites are pseudomorphs replaced by snow white baritocalcite. This replacement supposedly took place at an even later phase when calcium-rich fluids migrated through the formation. This theory is supported by the occurrence of floater quartz crystals in a pocket where, on the one side, the quartz aggregates display the luster of ‘bergkristall’ and are coated on t...


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The History behind the Mike Lurie Collection

May 23, 2017

The Lurie family lived in Bulawayo, Rhodesia/Zimbabwe, from 1951 to 1966. During this time Robert’s late father, Mike, worked as a manufacturer’s representative. His job took him by car all over Southern Rhodesia (Zimbabwe), Northern Rhodesia (Zambia) and Nyasaland (Malawi). Every now and again Mike would stop his car in the middle of the bush to take a break from the difficult, long distance driving. He would often notice something shining, or an agat...


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Working With Diamond

May 23, 2017

Duncan Miller

No, this is not about polishing diamonds, which in South Africa is illegal without a license, but about working with diamond grit or paste. For the coloured stone gem cutter, diamond paste is easier to source and to use. Loose grit and pastes are available in a range of mesh sizes, with crushed natural diamond or synthetic diamond. Synthetic diamond is made as single crystals and polycrystalline aggregates. The polycrystalline diamond breaks down with use to produce finer parti...


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TOURMALINE

April 25, 2017

Duncan Miller

Tourmaline can be temperamental. Rough tourmaline occurs in two distinct shapes – globular nodules and elongated pencil-like crystals elongated in the direction of the c-axis. The globular nodules sometimes spall concentrically, like onions, and the pencils sometime fracture transversely. This behaviour is difficult, if not impossible to predict, although fine cracks in the ‘skin’ of tourmaline pencils is not a good sign. The cracked skin must be removed by preforming or th...


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