The basic properties of opal, such as density, hardness, and refractive index have been known for a long time, and are recorded with some accuracy even in the 19th century textbooks. The variability of density and refractive index has also been recognised, and generally correctly attributed to the variable water content of the specimens. This was certainly valid for those samples of opal, common or precious, which were essentially transparent or translucent, and contained little 'impurity' elements such as iron.

The density of opal is recorded in the 1932 edition of Dana's A Textbook of Mineralogy (R1772) as ranging from 1.9 to 2.3, or, 'when pure', 2.1-2.2. The measured density of some opals may be influenced by the porosity and permeability of the particular sample; some opals can absorb water rapidly, and the density figure will be influenced by this property. Compact varieties will not, as a rule, absorb appreciable water during a density determination, and will yield density figures in the range of 1.99-2.25 (R1607).

Kokta (R1582), in a paper published in 1930, gathered together earlier work where density had been measured as an adjunct to chemical analysis, and was able to show quite clearly the relationship between water content and both refractive index and density, providing there were not too many other constituents present. He states:

Numerous determinations are quoted by Kokta, perhaps the earliest being 1826 by R. Guillemin; this density is of interest:

Kokta himself examined 18 opals, determining their water content, density and refractive indices. A plot of his determinations of water content against density, as well as data quoted from earlier workers is illustrated. It appears that density decreases slightly with increasing water content; the scatter of the individual determinations could be caused by the presence of heavier elements such as iron, by different porosities of samples, and whether density determinations were made in water or organic liquids.

The hardness of opal, as measured on the empirical Moh's scale, is usually recorded as about 5.5-6.5 for the compact varieties. As with the density, the hardness will vary somewhat according to the physical structure of the sample. The more porous examples, especially those which readily absorb water, will tend to lie towards the lower limit of the range, while the more translucent materials with conchoidal fracture will lie in the higher range. Precious opals, no doubt due to their special mode of formation and internal structure, tend to lie in the region 5.5-6. The earthy varieties have a much lower apparent hardness; diatomites (kieselguhr) can be crumbled between the fingers.