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