|This zircon gemstone came from Sri
Lanka, the world's major source of green zircon (along with Burma). On the Moh's
hardness scale it ranks 6.5 to 7.0.
Chemically, zircon it is ZrSiO4 . It typically includes a variety of impurities, e.g., hafnium, iron, thorium and uranium. This example does not appear to have any significant quantities of thorium, but it has sufficient uranium for its radioactivity to be easily detectable with a survey meter.
The radiation damage brought about by the presence of the uranium (and thorium if present) ultimately converts the crystals to a glass. Gemstones exhibiting such damage are said to be in a metamict state and the green color is typical of such a stone.
Most gemstone quality zircons have been heat treated, but I do not know if this
has been done with this example.
Size: 10.5 x 7.2 mm
Weight: 0.52 grams; 2.7 ct.
Activity: ca. 1,300 pCi (50 Bq) of U-238 with all the members of the decay series in secular equilibrium.
ca. 400 cpm above background with a pancake GM
Copyright 1999, Oak Ridge Associated Universities