I am trying to id a gemstone, it is a teal colored octagonal cut stone and it in fact looks like blue green tourmaline:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=290387533622#ht_1616wt_883

but what I am facing problems with is that the stone only emits light in one direction. if you look at it head on it is blue green, but if you look at the side, no light is visible at all, no matter how bright the light is. now i know that tourmaline is pleiochroic but due to the fact that the stone turns opaque i was wondering if this was just a weird pleiochroism or something else so i was wondering if anyone knew of any minerals that behaved this way

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