Noncrystalline hydrous silicon dioxide, This is produced by a lengthy and difficult process and, in most cases, corresponds exactly to natural opal. Sometimes, however, the particles of silica are impregnated with plastic instead of silica gel, the resulting product thus having a different com-position, despite being identical in appearance.
Although it is always sold as ready-cut stones, it is presumably manufactured in small sheets a few millimeters (probably 6 or 7) thick. It has lively iridescence, like that of precious opal. The other varieties of opal are of no commercial interest and are therefore not manufactured.
The density varies from 2.00 to 2.14 g/cm3, but can be as low as 1.95 for pieces impregnated with plastic. The refractive index is about 1.45 The hardness of 4.5 to 5 is a bit lower than that of natural opal. It has even greater porosity than natural opal, although this is eliminated in the types impregnated with plastic.
Initially, it was only produced in France, but it is now manufactured in Japan as well.
49.1 Black and white synthetic opal
Synthetic opal came on the market in 1974 and is produced in the two basic varieties of ornamental value, with a light or dark ground.
It matches the appearance of noble or precious opal precisely, with patches of iridescence on a milky white or deep blue to smoke gray or blackish ground. Naturally, the contrast between the background and colored patches produced by light diffraction is stronger in the dark variety. It is generally cut into oval or round cabochons; more rarely, into less regular shapes, the thickness not usually exceeding 5 millimeters. Recently, however, polished, spherical necklace beads with a diameter of 7-8 millimeters and more have been manufactured, proving that it can be produced in greater thicknesses.
It is very hard to distinguish from natural opal. Only under a microscope can one see a difference in the patches of color, which have a minutely tesselated appearance like lizard skin, never seen in the natural variety. As a result, the edges of the colored patches are also minutely sinuous and indented.
The white opal is less expensive than the black, which costs as much as some natural secondary gems, but less than the most expensive synthetic products.
Writer – Curzio Cipriani & Alessandro Borelli