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
ZrSiO_{4}, 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 _zarqūn_,
vermilion, or the Persian _zarqūn_, 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 ὑάκινθος, 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.
[Illustration: 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.