OPAL

DESCRIPTION

Opal has been one of the favoured precious gemstones for many centuries. Opals are amorphous crystals of hydrated silicon dioxide ( SiO₂nH₂O) where the water content can reach up to 8 % . The precious opal stones are brittle, heat sensitive and break and scratch easily. Opals of Australian origin are the best known, most of them exhibit brilliant play of colours emanating from inside. To bring out the optical effect opals are generally shaped as cabochons. The Mexican variety of opals is generally colourless and clear, although the best known Mexican ‘fire opal’ is of orange or reddish colour. Another variety is black opal which is rare and very beautiful, because the black background enhances the diffraction colours. Generally opals do not show any crystalline X-ray diffraction pattern, signifying that they are structurally amorphous.

We have two kinds of synthetic opal available. The first one is an opal imitation, the plastic impregnated opal. It is available in pieces weighing 20-100 g. The other variety is synthetic opal, the chemical composition and physical properties of which are quite similar to the Gilson Synthetic Opal. However, it does not have the “lizard skin” effect - a specific identification property for the Gilson Synthetic Opal. Synthetic opal is available in pieces of 2-40 g.

PROPERTIES

Created (plastic impregnated) Opal:

Chemical compositions:	silica-70-80%, rezin-20-30%
Crystal system:	Amorphous
RI:	1.43-1.46
Hardness:	4,5-5.0
Specific Gravity:	1,8-1,9

Synthetic Opal:

Chemical compositions:	silica-97-99%, zirconia-1-3%
Crystal system:	Amorphous
RI:	1.46-1.47
Hardness:	6.0
Specific Gravity:	average 2.15

HISTORY

As well as occurring naturally, opals of all varieties have been synthesized experimentally and commercially. The synthesis of opal has been a tough task and is only a recent achievement. The problem was that the origin of colours in precious opals remained a mystery for quite a long time. This puzzle has attracted the attention of some of the greatest physicists, like C. V. Raman, who hypothesized that opal should have a multiple layered structure of silica material having a slightly different refractive index. While this explanation is almost close to the truth, a definitive and detailed answer was given in 1964 by J. V. Sanders and his co-workers in Australia , who carried out an electron microscopic study of opals. They established that there is a close-packed arrangement of silica spheres of uniform size inside the SiO2-n-H2O matrix. The empty spaces between the spheres are also regular. These regular arrangements create a diffraction lattice, which produces the ‘opal effect’ and the coloured reflections similar to Bragg reflections of visible light. The successful synthesis of opal in the laboratory confirms this explanation.

The discovery of the ordered sphere structure of precious opal led to its synthesis by Pierre Gilson in 1974. The synthesized material is distinguishable from natural opal by its regularity. In Gilson's opals, the patches of colour are seen to be arranged in a "lizard skin" or "chicken wire" pattern under magnification. Synthetics are further distinguished from naturals by the former's lack of lasting fluorescence under UV light.

Nowadays a lot of so-called synthetics, however, are more correctly termed imitations, as they contain substances not found in natural opal (e.g., plastic stabilizers). The Gilson opals often seen in vintage jewellery are actually an imitation consisting of laminated glass with bits of foil interspersed.

SYNTHESIS

The synthesis of opal is very complex. Three steps necessary to produce synthetic opal are (i) to form silica spheres of exactly the same size, (ii) allowing them to settle into a close-packed arrangement, and (iii) to fill the interspace with the material of appropriate refractive index, with SiO2-n-H2O combination. While the first two steps are relatively easy to carry out, the third and final step involves the use of right pressure and temperature combination to make the final product. There are many procedures developed to synthesize opal, one of the possible procedures is outlined below.

In the first stage, the synthesis of monodisperse particles of silica in alcoholic sols is carried out. The particles are formed by the hydrolysis of tetraethyl ester of orthosilicic acid, Si(OC₂H₅)₄, or TEOS, in ethanol. The catalyst for this reaction is ammonia. Small nuclei are formed first from the liquid phase. Then the growth of the silica particles takes place. By adding further amounts of TEOS, the particles of silica grow up to the diameter desired.

In the second stage, the silica particles are precipitated either by spontaneous sedimentation, or by centrifuging. In the third stage of the synthesis, the precursor opal has to be dried in order to remove liquid from its pores. Subsequently, this substance (dried precursor opal) must be strengthened by thermal treatment in a furnace. After this treatment, the opal substance becomes a fragile, non-transparent material that has an open porosity about 35 per cent.

For the production of synthetic opal, it is then necessary to fill pores in the opal substance with a silica gel. In this stage proper consideration must be given to the difference in the indices of refraction that occur between the gel and the monodisperse particles in the forming opal.

COLOURS AVAILABLE

Synthetic opal: crystal, white, black.

USES:  Synthetic opal is a valuable raw material to be used in jewellery.