GemSelect Newsletter - January 2008
You'll often see advertisements for tanzanite, touting it as the gemstone that is "585 million years old". It's not clear exactly why that is a selling point for the gemstone buyer, since most minerals are hundreds of millions of years old. But if you're interested in a truly ancient mineral, look at zircon. 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 zircon gems represent the earliest record of the Earth's crust. 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 zircon in the form of tiny grains. 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 signs 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 was previously thought. According to previous theories, the first cells with a nucleus appeared about 1.5 billion years ago.
Zircon is thus a tremendously important mineral. For gemologists, zircon is fascinating. It is a remarkably dense mineral; about 50% denser than diamond. That means if you have a diamond and a zircon gemstone of similar size, if the diamond weighs 1 carat, the zircon will weigh about 1.5 carats. Therefore, the dimensions of any zircon gems should be carefully checked before purchase, since the stone may be smaller than expected.
Zircon also has very marked birefringence. Birefringence is a measurement for gemstones that 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 such gems. Zircon has such high birefringence that a zircon gemstone may appear fuzzy or display 'facet-doubling' when it is viewed through the table. Special cutting skills are
required to minimize these effects.
In addition to its notable birefringence, zircon has a very high refractive index (1.810 to 2.024), which is somewhat close to that of diamond (2.417 to 2.419). Zircon also has impressive dispersion; the tendency to split white light into the spectral colors. This means 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. Zircon should not be confused with cubic zirconia, a well known diamond simulant. Cubic zirconia 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.
Each month we focus on one of the lesser known gemstones. This month's featured stone is chrome diopside.
Until recently, fine chromium-bearing green gemstones were retricted to emerald, tsavorite garnet and chrome tourmaline. All were very expensive. In 1988 in Russia, there was a major find of chrome diopside, a chromium-rich variety of calcium magnesium silicate. The saturated green of diopside is easily the rival of tsavorite and chrome tourmaline, but the price is substantially lower. The only limitation is that chrome diopside is a softer stone (with a score of 5 to 6 on the Mohs scale), which means that it's not suitable for everyday rings. But it is very well suited for earrings and pendants, and looks stunning when set in gold.
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Happy Gem Hunting!
Your friends at GemSelect
- First Published: January-01-2008
- Last Updated: October-24-2017
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