|Zircon and Zirconology
Most gemstones were formed in the earth millions of years ago. But the mineral zircon is a special case. The oldest known zircon samples, from Western Australia, are more than 4.4 billion years old.
Zircon is actually the oldest known mineral on earth. Considering that the earth formed 4.56 billion years ago, the oldest zircons represent the earliest record of the earliest crust of the earth. Zircon was the first crystal to form in molten granite as it cooled to form rock. The ancient granitic rock has long eroded away; the only record that is left is tiny grains of zircon. This means that zircon is in fact the oldest thing on earth; the oldest samples are even older than the moon, which formed about 4 billion years ago.
Zircon is of such interest to geoscientists that it has spawned a discipline of "zirconology." Scientists have discovered that zircon contains trace elements of uranium and thorium, elements with known half-lives. Since uranium decays to lead at a known rate, radiometric dating can be used to estimate the age of zircon. Scientists have also analyzed zircon crystals for oxygen isotopes and have found the telltale signature of rocks that have been touched by water: an elevated ratio of oxygen-18 to oxygen-16. This has led to a rethinking of the origins of life on earth. If there was water 4.4 billion years ago, then life may have existed on earth much earlier than previously thought. On previous theories, the first cells with a nucleus appeared about 1.5 billion years ago.
Zircon is thus a tremendously important mineral scientifically. But for gemologists zircon is also fascinating. It is a remarkably dense mineral, about 50% denser than diamond. That means if you have a diamond and a zircon of similar size, if the diamond weighs 1 carat, the zircon will weigh about 1.5 carats. So be careful to check the dimensions of any zircon you buy or the stone may be smaller than you expected.
Zircon also has very marked birefringence. Birefringence is a measurement for gemstones which are doubly refractive; that is, gems which have two different refractive indices. When a beam of light enters a doubly refractive gem, it is split into two beams, each travelling at a different speed and on a different path through the crystal. Birefringence is a measurement of the difference between the two refractive indices in gems. Zircon has such high birefringence that a zircon may appear fuzzy or display 'facet-doubling' when you look down through the table. It requires special cutting skill to minimize these effects.
In addition to its notable birefringence, zircon has a very high refractive index (1.810 - 2.024), approaching that of diamond (2.417 - 2.419). Zircon also has impressive dispersion, the tendency to split white light into the spectral colors. What all this means is that zircon is an exceptionally brilliant gem. It's not surprising that in the days before diamond simulants, white zircon was often used as a substitute for diamond. But don't confuse zircon with cubic zirconium, a well known diamond simulant. Cubic zirconium is a recent invention of man; zircon is the oldest natural thing on our planet. It's a stone that every gem collector should have in his or her collection.