Zircon Gemstone Information
While zircon in fashioned form can be found in a wide range of colours the crystals are usually reddish brown and heat treatment is needed to produce the colourless, golden yellow and bright blue colours most seen in jewellery. Other colours, often very pleasing, might be called fancy zircons as the range is considerable. We should understand from the first that some zircons are much older than others and, like older people, behave in different ways. Over time some zircons have suffered a breakdown of their crystal lattice to the extent that in extreme cases they are to all intents and purposes amorphous. They show little or no birefringence and their physical properties are different from those of other zircons. They are ‘low’ zircons and come virtually exclusively from the gem gravels of Sri Lanka. Zircons from elsewhere, with their crystal lattice intact, are ‘high’ zircons. The name metamict (first used by Brögger in 1893) is applied to low zircons, which approach a glassy amorphous state, are
almost always green and can be radioactive though gem specimens do not normally show this property to any significant extent.
Zircon is zirconium silicate ZrSiO4 and forms prismatic crystals of the tetragonal system with square cross-sections and is sometimes attractively terminated by pyramids; large reddish brown crystals from Nigeria can be especially attractive. Geniculate twins are also found. Crystals show a markedly oily lustre. Most gem zircon occurs as pebbles in gem gravels. Zircon is often brittle and faceted stones have traditionally been kept separate from one another so that their facets do not become ‘paper worn’. Thai dealers for years used ‘zircon twists’ in which several stones were kept in screws of black paper.
The hardness is 7.5 and the SG 4.5–4.7. The RI for the ordinary and extraordinary rays is 1.925–1.961 and 1.980–2.015 respectively with a birefringence of 0.059, uniaxial positive. Metamict zircon may have SG around 4 and RI as low as 1.78 with scarcely perceptible birefringence. The dispersion, seen at its best in the heat-treated colourless material when faceted, is about 0.039. Properties to some extent identify the degree of metamictization and gem zircons have been classed as high, low and intermediate, the absorption spectrum often providing a useful
indication of a specimen’s position on this arbitrary scale. Zircon shows very weak pleochroism in general though blue heat-treated stones will give a blue and near-colourless response.
Heating these zircons may cause this band to sharpen and other lines to appear. There are some rare variations of the low-type absorption spectrum, one of which shows three broad strong bands in the red at 691, 669 and 653.5 nm, the centre band being the strongest. Another type, which curiously has been found only in zircons with an RI of 1.82 and an SG of 3.98, shows a vague band at 655 nm and another at 520 nm.
Some zircons show cathodoluminescence and thermoluminescence but the responses are not usually valuable in gemmological testing. Intermediate types may give an orange incandescence in a Bunsen flame – thorium has
been suggested as the cause. Neutron irradiation of yellow crystals causes their colour to change to bright green (the colour of metamict zircon).
Subjecting zircon to any form of irradiation may cause them to discolour. This is especially true of the colourless and blue specimens whose colour has been arrived at by heating. Though gemmologists speak of colour fading
this is, like so many other pronouncements, incorrect; dark patches appear in the stone and spread as the treatment continues. Heating usually restores their original appearance though not, it can be imagined until
some heart-searching has taken place.
Both the low and intermediate zircons when heated to about 1450 °C may achieve an SG of 4.7 with high-type RIs and sharper absorption lines. Heating the reddish brown Indo-Chinese crystals gives blue, colourless or yellow specimens according to whether or not an oxidizing atmosphere is employed. Colourless and sky-blue zircons need
reducing conditions while golden yellow colours are achieved in oxidizing conditions. Gem zircon is usually the high type. The best account of treatment can be found in Nassau, Gemstone Enhancement, second edition, Butterworth Heinemann, Oxford, 1994; ISBN 0750617977. Nassau reminds readers that many references to the
methods used in heating zircon can be found in Eppler, Das Geheimnis des Zircons, in Goldschmiede Zeitung 51, 531 (1936) and by Buckingham in the Journal of Gemmology 2, 177 (1950).
Metamict zircon shows the most characteristic inclusions. Most prominent are tension fissures meeting at an angle of 57°5 and probably due to the degeneration process. They echo the original crystal’s prism and
dipyramidal faces. Also due to isotropization are disc-shaped tension fissures. Unequal isotropization may give rise to parallel stripes which also suggest tetragonal form (Figure 7.3). Ilmenite may be found in fractures
and some healing fissures are reported.
Zircon is unusually common and widely distributed though most gem crystals occur in the gem gravels of Sri Lanka and some are found in the Mogok area of Myanmar and in Nigeria. The red zircon of Expailly, Auvergne, France, has already been mentioned. Gem crystals in a variety of colours and sizes have been found in the Mud Tank area of the Harts Range, Northern Territory, Australia. These crystals appear to have undergone
less structural damage than the Sri Lankan material. Fine crystals are reported from Slyudanka, Irkutsk Oblast, Russia, and from Seiland Is., Alta Fjord, northern Norway; this area has produced beautiful orange crystals
up to 2.5 inches in length.
The gem zircon locations in South-east Asia include the Pailin districtof Kampuchea and the Chiang Mai district, Thailand. The zircon producing sites overlap political boundaries in many cases so that names can be hard to locate. In all cases material is sent to Bangkok for fashioning.
Reddish brown zircon is reported from decomposed alkali basalts near Jemaal Kaduna, Nigeria. This is a source of fine crystals.