We are not concerned here with the special characteristics of precious opal. The development of colour in the gem variety is dealt with in the section The Colour of Opal. One of the main optical properties of the whole family of opal, and of transparent minerals in general, is the refractive index.

Mineralogists have traditionally determined this property of non-opaque minerals and other materials by immersion of a powder in liquids of known refractive index (the immersion method). Until the advent of X-ray diffraction (XRD), the determination of optical properties, especially refractive index, was the most useful method of identifying minerals. Even today, it can help to distinguish between minerals of similar structure which give similar XRD patterns.

Using the immersion method, the refractive index can be determined for most materials to three decimal places. For isotropic materials (those with only one refractive index) it is possible, with care, to measure the index to a fourth decimal place.

When enough material was available, prisms could be cut which enabled the refractive indices to be determined with great accuracy using optical instruments. The optical mineralogists of the 19th century frequently quoted such figures, and there is every reason to believe that their determinations were substantially accurate.

In relation to opal, Kokta (R1582) collected published data from earlier literature in order to see if there was a correlation between the composition, especially the water content, of opals, and their refractive indices. There were in fact, few investigations relating water content to optical properties.

Kokta himself measured the refractive indices on the 18 opals for which he determined water content and density, and showed that there was a broad relationship - refractive index increasing with decreasing water content. His measurements are plotted as a graph. The scatter of the points is probably due to the differing physical characteristics of the samples mentioned earlier, and perhaps the presence of 'impurity' elements.

It can be seen that the refractive indices of most opals lie within a fairly narrow range between 1.44 and 1.46, although occasional samples with high water contents will have lower indices.

Another property of non-cubic non-opaque crystalline materials is that called double refraction, or birefringence. Such materials have the property of splitting a transmitted light beam into two polarised rays.

This phenomenon is observed when a thin section (a thin slice, usually 30 µm, or 0.03 mm thick footnote, mounted on a glass slide) of the material is viewed in a petrological microscope between crossed polarisers. The polarised rays of the transmitted light cause interference which are seen as white or coloured effects in the crystals of the thin section. Cubic and amorphous materials do not show this phenomenon; when viewed in the same manner they remain dark, although precious opal is an exception to this rule.

Some common opals show this phenomenon. As indicated in the topic, Classification, most common opal is crystalline (opal-CT). In most cases the crystallites are too small for birefringence phenomena to be seen. However, sometimes the crystallites have grown sufficiently large for birefringence effects to be seen. These usually show that the opal is composed of tiny, fibrous crystals

This structure was first described by Mallard (R1589) in the late nineteenth century; he called such opal by the name of 'lussatite'. This term is reappearing in the literature today, being recognised as a useful terminology.

A property of precious opal which is seldom recognised is its propensity to show a pseudo-birefringence in thin sections, especially if the slice is made slightly thicker than usual. When viewed in the microscope between crossed polarisers, bright colours similar to those found in some crystalline minerals are seen.

This is caused because some of the light passing through the opal is diffracted, but at the same time, it is polarised. When the plane of polarisation of the diffracted beams is not parallel with the axes of the polarisers of the microscope, the coloured rays can pass through and be seen by the observer. Any non-diffracted light is extinguished by the crossed polarisers, thus increasing the apparent intensity of the coloured rays.