Duncan Miller

It’s play time!  For this exercise you need a flat computer screen with an open blank Word page, a smallish clear quartz crystal, a quartz sphere if you have one, or if not, some clear quartz beads (glass beads won't work), plus a pair of cinema 3D glasses or Polaroid sunglasses. 

Quartz crystals are anisotropic. This means that a ray of light travelling through the crystal is split into two polarised rays, vibrating at right angles to each other. There is only one direction of travel where this does not happen, and that it parallel to the c-axis, the long dimension of a well-formed, prismatic quartz crystal. Obviously, this is easy to determine in a well-formed crystal, but how can we determine the c-axis, or optic axis of a quartz sphere, gemstone or an irregular piece of rough?

You can do this without fancy equipment. All you need is a flat computer screen, an old pair of polarising 3D movie spectacles or Polaroid sunglasses, and a magnifying 10× loupe. Look through the front of the spectacles at a white computer screen and rotate the spectacles. At some position the light coming through the lenses will be cut out and go dark. This is because your white computer screen produces plane polarised light which is blocked by the lenses when their polarisation direction is at right angles to that of the screen. Now, holding the spectacles in their dark position in one hand, with the other rotate a small quartz crystal between the screen and one lens. You will see it go dark and light alternately in most directions. But if you look down the length of the crystal it will remain dark when you rotate it around the c-axis. This sounds complicated but it is easy to do.

Now, if one does the same thing with a quartz sphere, it too will go dark and light at various positions. But as you experiment with it you will find that in one position the dark areas form a cross, with a light centre surrounded by coloured rings (Fig. 1). This so-called ‘bull’s eye’ interference figure means you are looking along the c-axis or optic axis (Fig. 2). Unfortunately, you won’t see this effect with all quartz spheres, only those cut from untwinned quartz. Twinning can disrupt the interference figure. What you will see when looking along the c-axis may be a more confused arrangement of colours (Fig. 3).

Figure 1. Old 3D movie spectacles held in front of a white computer screen in the dark ‘extinction’ position, with a small quartz sphere held between them. It has been rotated so that we are looking along the c-axis, or optic axis, of the sphere. This produces a characteristic ‘bull’s eye’ interference figure consisting of a dark cross with a light centre.

Figure 2. A centred uniaxial interference figure in quartz, taken through a polariscope, using the same quartz sphere as in Fig. 1. This ‘bull’s eye’ pattern of a dark cross with an empty centre surrounded by concentric coloured rings is characteristic of quartz.

Figure 3. Not all quartz spheres are so well-behaved. This slightly larger sphere was cut from Brazil twinned quartz. This produces a rather confused interference figure, with four darker patches and coloured fringes rather than a ‘bull’s eye’ pattern when looking along the optic axis under crossed polarisers.

The photographs in Figures 2 and 3 were taken through a polariscope (Fig. 4). This is a device with a light in the base, a lower polarising filter, and an upper polarising filter that can be rotated to the dark, or ‘extinction’ position. It is more convenient to use than a pair of old 3D movie spectacles, but many times more expensive. This instrument is very useful for determining the optical orientation of gem rough, but plenty of information can be obtained from using polarising filters from 3D movie spectacles (see ‘How to play with polarized light’ at https://homepage.rub.de/olaf.medenbach/eng.html and also https://www.exploratorium.edu/snacks/polarized-light-mosaic).

Figure 4. A polariscope consists of a light below a lower polariser, a space in which to manipulate a specimen, and an upper polariser that can be rotated to achieve the dark, extinction position when the two polarisers are ‘crossed’. It is fairly easy to construct one using two polarising filters from 3D movie spectacles.

The property of light splitting into two rays polarised at right angles to each other when travelling in all directions in quartz except parallel to the optic axis is called birefringence. It can give rise to doubling of an image viewed through the crystal. This double refraction can also be seen in some orientations looking at rutile in a large rutilated quartz crystal (Fig. 5). This optical doubling disappears when you look along the optic axis, and also at right angles to it. This is because light travelling at right angles to the optic axis in quartz does split into two rays vibrating at right angles to each other, but they travel in the same direction. However, they will not produce the characteristic optic axis interference figure, so this direction can be distinguished from the c-axis using crossed polarisers.

Figure 5. This photograph of a 56 mm diameter quartz sphere clearly shows the doubling of the image of some of the rutile inclusions, causing them to look like parallel pairs of needles. This is due to the birefringence of the quartz.

Figure 6. Colourful interference colours under crossed polarisers seen looking down the optic axis (c-axis) of facetted synthetic quartz. This stone was cut with the table facet at right angles to the c-axis.

To find the optic axis in quartz rough and gemstones you need one additional element – a magnifying glass to act as a lens in order to see the interference figure. First you rotate the stone between crossed polarisers, looking for interference colours (Fig. 6). Then you know you are looking long the c-axis of the stone. A 10× magnifying lens needs to be inserted above the stone to resolve the interference figure. (A quartz sphere acts as its own lens.)

This requires more than two hands, so a bit of ingenuity is needed for something to hold the 3D spectacles in place in the correct orientation, or a more elaborate set-up with a polariscope, lens and camera as in Fig. 7. With this arrangement you can see not only the characteristic ‘bull’s eye’ interference figures of quartz, but interference figures of other minerals too (Fig. 8).

Figure 7. Polariscope and hand-held magnifying loupe used to photograph the interference figure of a rough scapolite crystal viewed down the optic axis (Fig. 8).

Figure 8. Slightly off-centre uniaxial interference figure of scapolite viewed down the optic axis (c-axis), taken with the set-up shown in Fig. 7. Compare this with the quartz interference figure in Fig. 2 and note the difference. In scapolite the dark cross reaches completely to the centre, which is normal for a uniaxial interference figure, with the exception of quartz. In quartz there is a central ‘bull’s eye’ or ‘hole’, which is completely typical of quartz and diagnostic if seen in either rough or cut gemstones.