Zircon

Zircon is not a very well-known gemstone, but it is one of the most important gemstones of today. Many people confuse zircon with the artificial diamond simulant, cubic zirconia. Of course, the two are completely different – cubic zirconia (CZ), is a lab-created synthetic material, while zircon is a naturally occurring gemstone with a very different chemical composition. However, before the introduction of diamond simulants like cubic zirconia and moissanite, white zircon was actually one of the most popular diamond substitutes, owing to its high dispersion and refractive index, and was often marketed under the misleading trade name of ‘Matura diamond’. Zircon is also remarkably dense and exhibits a very pronounced level of birefringence. Zircon has such high birefringence that it may appear fuzzy or display facet-doubling.

Zircon occurs in a wide range of colours, of which white or colourless is likely the least valuable or important. The most popular zircon today is blue zircon, usually occurring with green pleochroism, which can result in interesting teal-like colors. Blue zircon is actually produced by heating more commonly occurring brown zircon. However, only some brown materials have the appropriate physical structure to turn blue when heated, typically only deposits found in South East Asia – which is why most blue zircon comes from Cambodia or Burma. Blue zircon is one of the traditional modern birthstones for December.

Zircon is a tremendously important mineral, indeed its prominence led to the emergence of the discipline of ‘zirconology’ in gemology. It is the oldest mineral on Earth, with samples found in Australia that are over 4.4 billion years old. Scientists discovered that zircon actually contains trace elements of uranium and thorium, both of which are elements with known half-lives. It is the uranium and thorium that cause such tremendous variations in the physical properties of zircon. Zircon is typically classified as low, medium and high zircon, in reference to the presence of optical properties.

Low zircon has higher traces of radioactive uranium and thorium, which causes an alteration in physical structure, rendering it near-amorphous, though its physical appearance remains the same as high or medium zircon. Low zircon is typically green to brownish in colour. Green zircon is rarely seen in the gem trade and is highly sought after.

Zircon is a zirconium silicate by composition. An unusual trait with zircon is that its gemological properties exhibit very wide ranges, such as hardness ratings spanning from 6.5 to 7.5 on the Mohs scale. Zircon can also be easily distinguished by its notable birefringence; when looking down through the table of a stone, the doubling effect of facet edges can be easily seen. It also has a relatively high refractive index ranging from 1.810 to 2.024, and a density that can range between 3.93 and 4.73. Zircon also has a relatively high dispersion rating, exhibiting fire and brilliance comparable to that of fine diamond. Many stones will also exhibit strong pleochroism.

Zircon is actually the oldest known mineral on Earth; the oldest samples are even older than the moon, which formed about 4 billion years ago. Zircon was the first crystal to form in molten granite as it cooled to form rock. Low zircon formed as the result of a process associated with the presence of uranium and thorium. The natural radioactivity disrupts the crystal structure and produces the changes in color and density through a process known as metamictization.