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Where is it formed and where is it found?

• usually small and inclusion-rich
• Zircon is highly resistant to weathering (maybe the most resistant of any mineral) This, and its high specific gravity, favor its concentration in placer deposits and alluvial gem gravels.
• Gemstones have historically been recovered from many localities

Important characteristics:

• Easy to chip
• Doubly refractive with very obvious doubling of facets. A second example showing doubling of facets. High dispersion.
• Contains Uranium (U) and thorium (Th)
• undergoes radioactive decay and leads to damaged crysals ("Metamict") (appear cloudy)
• decay of U and Th occur at specific and well known rates. Measurement of isotopic abundances allows age dating of zircons ! (Thus zircons are very important minerals in the geological scheme of things.)

Treatments:

Metamict crystals + heat (1450 C) can restore crystallinity

Brown zircon + 800 - 1000 C in reducing environment -> colorless or blue zircon

Brown zircon + 800 - 1000 C in air-> colorless or golden yellow zircon 

Most colorless or blue crystals have been irradiated or heat treated!

• colorless : brilliant cut (colored stones often in emerald cut) to resemble diamond: you can distinguished by double refraction (which doubles the image of the facets), and by signs of wear

Note: UV (sunlight) can modify the color!

Heat treatment makes zircons more brittle, facets tend to abrade more

Detection: Geiger counter
Measurement: many scales:

• nCi = nanoCurie (billionth of a Curie)
Curie is the radioactivity of 1 g of radium
• mrem (thousanth of a rem)
rem = roentgen equivalent man (like a sunburn measure)
• rad = measure of absorbed dose: quantity of radiation able to deposit 100 ergs of energy)

360 mrem / year or 1 mrem/day

Safety limit: 5,000 mrem to entire body per year

Sc 46 or Ta 182 in blue topaz: ~ 25 nCi/g

Thus: for two identically sized stones: 

blue topaz (~ 25 nCi/g)

natural zircon (1.0 nCi/g of U238)

Does this matter?

• the average human contains 200 nCi of K40 and eats and drinks ~ 140 nCi of K40 / year
• the 2 nCi/ g from gems is insignificant.