Gem-Stones and Their Distinctive Characters

CHAPTER XXVI

ZIRCON

(Jargoon, Hyacinth, Jacinth)

ZIRCON, which, if known at all in jewellery, is called by its variety names, jargoon and hyacinth or jacinth, is a species that deserves greater recognition than it receives. The colourless stones rival even diamond in splendour of brilliance and display of ‘fire’; the leaf-green stones (Plate XXIX, Fig. 13) possess a restful beauty that commends itself; the deep-red stones (Plate XXIX, Fig. 14), if somewhat sombre, have a certain grandeur; and no other species produces such magnificent stones of golden-yellow hue (Plate XXIX, Fig. 12). Zircon is well known in Ceylon, which supplies the world with the finest specimens, and is highly appreciated by the inhabitants of that sunny isle, but it scarcely finds a place in jewellery elsewhere. The colourless stones are cut as brilliants, but brilliant-cut fronts with step-cut backs is the usual style adopted for the coloured stones.

Zircon is a silicate of zirconium corresponding to the formula ZrSiO₄, but uranium and the rare earths are generally present in small quantities. The aurora-red variety is known as hyacinth or jacinth, and the term jargoon is applied to the other transparent varieties, and especially to the yellow stones. The most attractive colours shown by zircon are leaf-green, golden-yellow, and deep red. Other common colours are brown, greenish, and sky-blue. Colourless stones are not found in nature, but result from the application of heat to the yellow and brown stones.

The name of the species is ancient, and comes from the Arabic zarqun, vermilion, or the Persian zarqun, gold-coloured. From the same source in all probability is derived the word jargoon through the French jargon and the Italian giacone. Hyacinth (cf. p. 211) is transliterated from the Greek ????????, itself adapted from an old Indian word; it is in no way connected with the flower of the same name. The last word has seen some changes of meaning. In Pliny’s time yellow zircons were indiscriminately classified with other yellow stones as chrysolite. His hyacinth was used for the sapphire of the present day, but was subsequently applied to any transparent corundum. Upon the introduction of the terms, sapphire and ruby, for the blue and the red corundum hyacinth became restricted to the other varieties, of which the yellow was the commonest. In the darkness of the Middle Ages it was loosely employed for all yellow stones emanating from India, and was finally, with increasing discernment in the characters of gem-stones, assigned to the yellow zircon, since it was the commonest yellow stone from India.

Fig. 78.—Zircon
Crystal.

Considered from the scientific point of view, zircon is by far the most interesting and the most remarkable of the gem-stones. The problem presented by its characters and constitution is one that still awaits a satisfactory solution. Certain zircons, which are found as rolled pebbles in Ceylon and never show any trace of crystalline faces, have very nearly single refraction, and the values of the refractive index vary from 1·790 to 1·840, and the specific gravity is about 4·00 to 4·14, and the hardness is slightly greater than that of quartz, being about 7¼. On the other hand, such stones as the red zircons from Expailly have remarkably different properties. They show crystalline faces with tetragonal symmetry, the faces present being four prismatic faces mutually intersecting at right angles and four inclined faces at each end (Fig. 78). They have large double refraction, varying from 0·044 to 0·062, which is readily discerned in a cut stone (cf. p. 41), and the refractive indices are high, the ordinary index varying from 1·923 to 1·931 and the extraordinary from 1·967 to 1·993. Since the ordinary is less than the extraordinary index the sign of the double refraction is positive. The specific gravity likewise is much higher, varying from 4·67 to 4·71. The second type, therefore, sinks in molten silver-thallium nitrate, whereas the first type floats. The second type is also slightly harder, being about 7½ on Mohs’s scale. By heating either of these types the physical characters are not much altered, except that the colour is weakened or entirely driven off and some change takes place in the double refraction. But between these two types may be found zircons upon which the effect of heating is striking. They seem to contract in size so that the specific gravity increases as much as three units in the first place of decimals, and a corresponding increase takes place in the refractive indices, and in the amount of double refraction. The cause of these changes remains a matter of speculation. Evidently a third type of zircon exists which is capable of most intimate association with either of the other types, and which is very susceptible to the effect of heat. It may be noted that stones of the intermediate type are usually characterized by a banded or zonal structure suggesting a want of homogeneity. The theory has been advanced that zircon contains an unknown element which has not yet been separated from zirconium. Zircon of the first type favours green, sky-blue, and golden-yellow colours; honey-yellow, light green, blue, and red colours characterize the second type; and the intermediate stones are mostly yellowish green, cloudy blue, and green.

It is another peculiarity of zircon that it sometimes shows in the spectroscope absorption bands (p. 61), which were observed in 1866 by Church. Many zircons do not exhibit the bands at all, and others only display the two prominent bands in the red end of the spectrum.

Of all the gem-stones zircon alone approaches diamond in brilliance of lustre, and it also possesses considerable ‘fire’; it can, of course, be readily distinguished by its inferior hardness, but a judgment based merely on inspection by eye might easily be erroneous.

According to Church, who has made a lifelong study of zircon, the green and yellowish stones of the first variety emit a brilliant orange light when being ground on a copper wheel charged with diamond dust, and the golden stones of the intermediate type glow with a fine orange incandescence in the flame of a bunsen burner; the latter phenomenon is supposed to be due to the presence of thoria.

The leaf-green stones almost invariably show a series of parallel bands in the interior.

Zircons vary from 5s. to 15s. a carat, but exceptional stones may be worth more.

By far the finest stones come from Ceylon. The colourless stones are there known as ‘Matura diamonds,’ and the hyacinth includes garnet (hessonite) of similar colour, which is found with it in the same gravels. The stones are always water-worn. Small hyacinths and deep-red stones come from Expailly, Auvergne, France, and yellowish-red crystals are found in the Ilmen Mountains, Orenburg, Russia. Remarkably fine red stones have been discovered at Mudgee, New South Wales, and yellowish-brown stones accompany diamond at the Kimberley mines, South Africa.

Clyx.com