Ekanite

Ekanite is an uncommon mineral notable primarily as being among the very few gemstones that are naturally radioactive. Most ekanite is mined in Sri Lanka, although deposits also occur in Russia and North America. Clear and well-colored stones command the best prices, but such stones are rare; the mineral's radioactivity tends to degrade the crystal matrix over time in a process known as metamictization. Furthermore, ekanite can easily be misidentified as tourmaline. The market for gemstone-quality ekanite is for the moment limited to collectors.
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General Information:
 Chemical Formula:
ThCa2Si8O20
 Composition:
(Molecular Weight = 856.87 gm)
Calcium 9.35 % Ca
Thorium 27.08 % Th
Silicon 26.22 % Si
Oxygen 37.34 % O 
 Empirical Formula:
ThCa2Si8O20
 IMA Status:
Approved IMA 1961
Physical Properties:
 Cleavage:
[101] Distinct
 Color:
green, yellow, or dark red.
 Density:
3.08
 Diaphaniety:
Translucent
 Hardness:
4.5 - Between Fluorite and Apatite
 Luster:
Vitreous (Glassy)
 Streak:
white
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General Information:
 Chemical Formula:
K(La,Ce,Th)2(Ca,Na)4(Si,Al)16O40
 IMA Status:
Approved IMA 1976
Physical Properties:
 Cleavage:
[???] Good, [???] Good
 Color:
greenish yellow.
 Density:
3.27
 Diaphaniety:
Transparent to Translucent
 Hardness:
4.5 - Between Fluorite and Apatite
 Luster:
Earthy (Dull)
 Streak:
white
More details...