Zircon is a relatively hard stone with a hardness of 7-7.5. Its high specific gravity (4 – 4.73), makes it easy to identify and few gemstones are as dense. Experienced buyers can identify zircons by heft alone. Because of its high refractive index, zircon can be exceptionally brilliant. It is also prominently doubly refractive with a high birefringence of .059. Because of this, faceted zircons are easy to identify because of the doubling of the facet junctions. Indeed the doubling is pronounced and the stones can be challenging to photograph as they may never appear focused.
Zircon has sustained the interest of gemologists for years owing to its variation of specific gravity and refractive indices. The composition of the mineral is a silicate of zirconium (ZrSIO4). Zircon forms mixed crystals with the uranium mineral coffinite and the thorium mineral thorite. In some specimens, the alpha particles from uranium and thorium have partly or wholly broken down the lattice of the zircon. Where the lattice has been broken down or degenerated from the crystalline state the stones are virtually amorphous and known as metamict. Depending on the degree of degeneration, some of the stones do not even respond as tetragonal minerals and under a polariscope will behave as singly refractive stones. Where the degenerative process has not begun (or may be little advanced, the term ‘high zircon’ is used). Intermediate stages between high and metamict zircon are also common.
Most interesting is the influence of heat on different types of zircon. Both the low and intermediate types tend to increase in density to the normal S.G. of around 4.70 upon heating to 1450 degrees C. The heating causes the dissociated silica to recombine as crystalline zircon.
Sri Lankan zircons are mostly yellow or green. The green stones are mostly of the low or intermediate type and upon heating to a dull red heat for about 1 hour they become much paler. However, the most important and dramatic heat treatment of zircons is carried out on the reddish brown crystals from Vietnam and Cambodia. The stones can be heated to an electric sky blue and the color is normally permanent. Some of these stones are also heated to white and used as substitutes for diamonds in some kinds of jewelry.
Other kinds of zircon notably the brownish honey colored stones from East Africa become a beautiful bright orange upon heating on a simple gas furnace for only one minute . Unfortunately, the color is not permanent and the stones will turn to a dingy yellowish grey after short exposure to sunlight. After a few days, these stone will revert to their original color.
Less known are the tenebrescent properties of some zircons. Also known as reversible photochromism, tenebrescence refers to the ability of minerals to change color when exposed to the natural radiation in sunlight. Studies of West African zircons dark brown zircons showed them to change to white upon high temperature heating. In sunlight the heated stones slowly became a dingy gray in a few days but then, after a few months in total darkness, they became very slightly pink. These stones are problematic to sell as one can never know what they will look like on any given day. Other tenebrescent minerals include hackmanite sodalite, spodumene, tugtupite, and natural yellow Sri Lanka sapphires.
Natural zircon is often confused with cubic zirconia, a laboratory-grown diamond stimulant. Natural zircon and cubic zirconia are totally unrelated, zircon is a naturally occurring gemstone while cubic zirconia is a synthetic.
Zircon is mined in Cambodia, Sri Lanka, Thailand, Myanmar, Australia, Vietnam, Tanzania, and Madagascar. The best blue zircons are from Cambodia and they have been heated to produce that color. The wide variety of colours of zircon, its rarity, and its relatively low cost make it a popular collector's stone.
Zircon is a stone of purity and innocence. It balances the emotions and enhances self esteem and unity. Was used as an amulet by travellers to protect against accident and injury on the journey. It was also used to stop the wearer from being stuck by lightening. Hindu poets tell of the Kalpa Tree, the ultimate gift to the gods, which was a glowing tree covered with gemstone fruit with leaves of zircon. Zircon has long had a supporting role to more well-known gemstones, often stepping in as an understudy when they were unavailable. In the middle ages, zircon was said to aid sleep, bring prosperity, and promote honor and wisdom in its owner. The name zircon is said to have been derived from the word zargoon which in Arabic means vermillion and in Persian gold-colored.
