The Zing of Zircon, Part 2: Curious Character and Causes of Colour

Zircon is somewhat unique among gemstones in that it occurs with a wide range of variable properties, namely its refractive index, birefringence, specific gravity, and mohs hardness. This variation is due to differences in zircon’s structure caused by radiation induced degradation. Zircon crystals typically incorporate the radioactive elements uranium (U) and thorium (Th), which then expose the zircon to constant irradiation from within. Both of these elements undergo radioactive decay at rates which are much too slow for them to have a significant effect on the human body in most contexts, but zircon crystals often survive for millions of years, allowing them to be sufficiently irradiated by the radioactive materials that they contain and undergo noticeable changes as a result; the primary effect that this ionising radiation has on zircon is the breakdown of its crystalline structure, resulting in an amorphous arrangement of zirconium silicate (ZrSiO4) in three dimensional space, a process known as “metamictization” or “metamiction”. 

Yellow zircon crystals on matrix from the Poudrette Quarry in Mont Saint-Hilaire, Quebec Canada; Image: IGS/ Modris Baum

As a zircon crystal is further damaged by radiation and becomes less crystalline, the refractive index, birefringence, specific gravity, and mohs hardness of the material decrease. In addition to changes in these properties, the apparent brilliance of zircon diminishes as its crystal structure becomes further damaged, and in many cases stones which are fully amorphous in structure will appear cloudy or hazy. Zircon which has completely lost its crystalline structure is described as “metamict”. Curiously, the dispersion value of zircon does not change in response to the metamiction process.  In the field of gemology, zircon gemstones are grouped into three separate categories or “types” based on the amount of metamiction they have undergone: “high” or “normal” type zircons are fully crystalline, “intermediate” or “medium” type zircons are partially amorphous, and “low” type zircons are considered to be fully metamict; the terms “alpha”,”beta”, and “gamma” are also sometimes used to describe these three categories. Subjecting zircon to high temperature treatment can cause stones to recrystallize, thus repairing radiation induced damage present in zircon’s crystal structure; in such cases medium type and low type zircons can be converted back to high type zircons.

One of many varicoloured gemstones, zircon gems are available in a wide range of hues. Orange, red, yellow, brown, green, blue, pink, peach, and colourless zircons are all found in today’s jewellery market, with blue, red, and green stones being among the most valuable.

2.09ct Radiant Master Cut Blue Zircon from Cambodia; Image: Skyjems

Zircon is regarded as an allochromatic gemstone, meaning that its colour is derived from impurities contained within a crystal rather than the intrinsic chemical structure of the mineral itself, but not much is known about what the precise impurities are that act as chromophores in zircon. It has been proposed that most stones which fall into the brown-peach-yellow colour range are coloured by isotopic defects in zircon’s native chemical structure (ZrSiO4) as many stones which display these hues are intermediate type zircon and have undergone natural irradiation, but it has also been proposed that some hues in this range are caused by an unknown ion impurity which has yet to be identified; in recent times, the terms “jargoon” and/or “jargon”, corruptions of the Persian “zargon”, will sometimes be used to describe yellowish or pale brown zircon gemstones. Green zircons are often low type zircons, with such stones being thought to get their colour from isotopic defects produced during the metamiction process. White zircons are exclusively high type zircons which have a completely intact crystal structure and contain no chromophoric impurities. The colour of red zircon may be the result of partial metamiction, as red stones are typically of the intermediate type, but it has been proposed that impurities of quadrivalent niobium (Nb4+) and/or quadrivalent rare earth elements, such as yttrium (Yt) and cerium (Ce), are the source of red zircon’s colour; it is likely that the chromophores responsible for red hues seen in some zircon gems are also behind the pink hues which zircon may exhibit. Presumably, orange zircon gemstones contain both the chromophore which causes red hues in zircon and the chromophore which causes yellow hues in zircon. Blue zircon is thought to be coloured by quadrivalent uranium (Ur4+, a.k.a. uranous ions) but this is still uncertain.  

Faceted zircon gemstones of various colours; Image: Gemological Institute of America

In the next part of this series read about zircon treatments, uncommon zircon varieties, and zircon simulants.

© Yaĝé Enigmus