Opal's Density

(via Australian Gemmologist, Volume 23, No.2, April – June 2007) Alan Hodgkinson writes:

The author describes how liquids such as sodium polytungstate and glycerine can be used practically to discriminate natural opals from synthetic and all-plastic imitation opals.

A typical gem quality opal has a water content of 6-10 percent and this is subject to both evaporation and drying out. In contrast, however, some opals are extremely hygroscopic, and upon immersion in water there can be considerable intake. This not only alters the weight and therefore the density of opal, but also can alter its appearance. As a consequence of this water intake, the whole stone looks more valuable. Also note how the opal has temporarily darkened where the water intake has concentrated.

Natural opal weight (before): 16.19ct
Natural opal weight (after): 18.10 ct

Obviously, any attempt at determining the specific gravity of such an opal, by hydrostatic or beam balance method would upset by its water intake. However, a useful response for estimating this opals specific gravity can be gained by the immediate reaction of the opal to immersion in sodium polytungstate. Webster gives the S.G for white and black and opal as near 2.1. Bear in mind, this density range is only significant when there is no matrix present. Please remember that the latter feature itself a pointer to a natural opal identity, unless the opal is part of a composite imitation. Those man-made opals with a 20 to 30 percent polymer infilling have a decidedly lower S.G in the region of 1.8 – 1.9, while those white synthetic opals that use zirconia as a sphere binder have a slightly raised S.G in the region of 2.2.

Suspension in sodium polytungstate
It is fairly simple matter to suspend an opal in the harmless medium, sodium polytungstate. This compound was demonstrated to the author in 1979 by Dr W W Hanneman. When pure, sodium polytungstate’s S.G is 2.8. Adding distilled water lowers its density. In contrast, heating the liquid evaporates and therefore raises its density. Once the opal is suspended, the R.I of the liquid can be confirmed by refractometer. This can then be read off against a straight line graph compiled by Hanneman which will indicate the approximate density of the opal. The alternative is to use known control stones (natural and synthetic) and compare the buoyancy or heaviness of the test opal against the behavior of the controls.

Glycerin as a heavy liquid
Glycerin is not harmful, washes off immediately with cold water, is not expensive, and might be found useful to separate opals from the all-plastic opal simulants, as pointed out by Emmanuel Fritsch. This viscous liquid has a density of 1.26 in which all imitation plastic opals will float. In contrast, all opal, whether natural or synthetic, will sink. Even those man-made opals which contain silica spheres but which have a substantial plastic binder, will sink. Apart from the plastic opal imitations, all of the above sink at variable speeds—dependent on size, and not density.