Opal Gemstones - A Collection of Historical Articles on the Origins, Structure and Properties of Opal

Various

• Publisher: Merz Press
• Year: 2015
• ISBN:
  9781473394124
• Topic: Biological Sciences
• Subtopic: Science General
• Index: Biological Sciences

This book contains classic material dating back to the 1900s and before. The content has been carefully selected for its interest and relevance to a modern audience. Carefully selecting the best articles from our collection we have compiled a series of historical and informative publications on the subjects of gemology and crystallography. The titles in this range include "Gemstone Manufacturing" "The Optical Properties of Gemstones and Crystals" "The Thirty-Two Classes of Crystal Symmetry" and many more. Each publication has been professionally curated and includes all details on the original source material. This particular instalment, "Opal Gemstones" contains information on their properties, origins, structure and much more. Intended to illustrate the main features of opal it is a comprehensive guide for anyone wishing to obtain a general knowledge of the subject and to understand the field in its historical context. We are republishing these classic works in affordable, high quality, modern editions, using the original text and artwork.

OPAL

By G. F. Herbert

Varieties: white-opal, black-opal, fire-opal,
water-opal, hydrophane.

Composition: SiO2+nH2O.

Colour: colourless (water-opal), when pure; milky
and various dull shades; reddish or orange (fire-opal).

Lustre: sub-vitreous to resinous.

Hardness: 5 1/2–6 1/2.

Specific gravity: 2·10.

Refraction: single.

Refractive index: 1·454.

THAT opal excited the keenest admiration in early days is evident from Pliny’s enthusiastic description of these stones: ‘Of all precious stones, it is opal that presents the greatest difficulty of description, it displaying at once the piercing fire of carbunculus, the purple brilliancy of amethystos, and the sea-green of smaragdus, the whole blended together and refulgent with a brightness that is incredible.’[1] From a passage a little further on in his work it appears that the faker even in those days was well acquainted with the possibility of counterfeiting opal by means of glass; Pliny proceeds to give a test for distinguishing the true from the false opal which can hardly have been infallible: ‘There is no stone that is imitated by fraudulent dealers with more exactness than this, in glass, the only mode of detecting the imposition being by the light of the sun. For when a false opal is held between the finger and thumb, and exposed to the rays of that luminary, it presents but one and the same colour throughout, limited to the body of the stone: whereas the genuine opal offers various refulgent rays in succession, and reflects now one hue and now another, as it sheds its luminous brilliancy upon the fingers.’[2] Owing to the foolish superstition which flourished during much of last century and still lingers even now, namely, that it is unlucky to wear opals, this gemstone has long lain under a cloud which began slowly to lift only when the black-opal was discovered in Australia. Indeed, the marvellous crimson glow which at a chance movement sweeps over a piece of opal, black with the darkness of night, is not the least striking phenomenon that modern jewellery has to offer.

The two chief varieties into which opal may be divided are white-opal and black-opal. Both terms are used in a wider sense than what would be understood by a strict interpretation of the words. The intrinsic colour of white-opal is always light, but it may be some tint, such as yellowish, other than white. A true black-opal is very rare, and most of this variety is dark grey or blue by reflected light. That the coloration is not due to ordinary absorption is shown by the fact that the transmitted and reflected beam have complementary tints; a blue stone, for instance, is yellow when viewed by light that has passed through it. In many pieces of black-opal the opalescent specks are too tiny to be worth isolating, and the whole iron-stained mass is cut and polished, and is sold as opal-matrix. The name of the species has reached us through the Latin opalus, from the Greek word ὀπάλλιος, which itself comes from a Sanskrit word upala, meaning precious stone, indicating that opals were first introduced to Europe from India.

Fire-opal is a variety which, as its name indicates, is reddish or orange in tint. It is fairly transparent and possesses only slight milkiness, but viewed in the appropriate directions it displays opalescence. Harlequin-opal has a variegated play of colour on a reddish ground. The term, girasol, also has been used for a fire-opal; it originated from the Latin words girare (turn) and sole (sun) and intrinsically has the same meaning as heliotrope, which has come from the Greek, but has a different significance. Gold-opal is used to-day as an alternative name for fire-opal. Wood-opal is, as its name implies, wood that has been petrified by opal. Hyalite, sometimes called Muller’s glass after its discoverer, is a remarkable opal which is as clear as glass; hence its name from the Greek word ὕαλος (glass). Cacholong is the bluish-white porcelain variety, very porous and adhering to the tongue, which is highly prized in the East; the name is adapted from kaschtschilon, a word of Tartar origin. Hydrophane, which is transliterated from the Greek word ὑδροφανής (apparent in water), is a curious form which has to be soaked in water before its opalescence is developed, the reason being that until water, which in refractivity is not far inferior to opal, penetrates into the cracks much of the light falling upon them is totally reflected at the nearer side and fails to cross them. It is, indeed, surprising how much water these hydrophanes can absorb.

In composition opal is silica, SiO2, with an amount of water varying from 6 to 10 per cent in precious opal. When pure it is colourless, but is nearly always more or less milky and opaque, or tinted various dull shades by the ferric oxide, magnesia, lime, alumina, etc., present as impurities. Opal differs from the rest of the principal gemstones in being not a crystal but a kind of solidified jelly, and it depends for its attractiveness upon its characteristic play of colour, which is in consequence known as opalescence. This play results from the fact that, as the jelly cooled, it no longer remained uniform in texture, but assumed the form of a series of extremely thin films, which differed, though slightly, in refractivity. Owing to this slight want of homogeneity these films act on light in the same way as soap-films, and give rise to similar colour phenomena, the thinner and more uniform the cracks the greater being the splendour of the display; exactly what colour is seen depends upon the thickness of the cracks and the direction in which they are viewed. In certain instances the mass is fissured with actual cracks, which contain air and are not filled up with solid material. As was explained above, such opals lack opalescence until the cracks have been filled with water. Some opal is almost water clear (water-opal), but nevertheless displays vivid opalescence when viewed in an appropriate direction.

DIAMOND-SORTING MACHINES

The refraction of opal is single and the values of the refractive index vary from 1·444 to 1·464. The lustre is sub-vitreous to resinous. The specific gravity ranges from 2·00 to 2·20; for the Australian opal it is about 2·12, and for the material from Mexico it ranges from 1·98 to 2·03. Opal is somewhat soft, varying from 5 1/2 to 6 1/2 on Mohs’s scale, and therefore is easily scratched.

Except that the fire-opal variety may be faceted, opal is usually cabochon cut either in the flat or the steep shape, generally the former in brooches and pendants and the latter in rings. Opals call for cautious handling. Many besides those called hydrophane will absorb some liquid, and their appearance may be spoilt, if they be dipped into dirty water, or ink or other coloured fluids be allowed to drip upon them. Owing to the peculiar nature of their structure opals are very susceptible to the effect of heating, and their appearance may be irretrievably damaged if they be heated.

The price of opal varies greatly, depending upon the intrinsic colour and the uniformity and brilliance of the opalesence. The usual rate per carat ranges from £1 to £4 for white-opal and from £3 to £15 for black-opal, according to size and quality; but a good dark stone, displaying a flaming opalescence, commands a fancy figure, since fine pieces of this kind are exceedingly rare. The value of the white-opal suffered a catastrophic fall upon the discovery and establishment of the black-opal. To the author’s knowledge, a lady, who as a bride had received from her husband ...

Table of contents

• INTRODUCTION TO GEMMOLOGY
• PRECIOUS OPAL
• THE OPAL
• OPAL
• OPAL
• OPAL
• OPAL
• OPAL
• OPAL
• OPAL
• OPAL