Gemstone Identification: Oily Luster - Topaz, Tourmaline, Peridot, Tanzanite

Question
I'm doing research for a contest I'm entering and I have found all the answers but one.  It is proving to be confusing.  I hope you can help me. The description of the gemstone is "OILY IN LUSTER, THIS GEMSTONE COMES IN COLORS OF GREEN, GREENISH-YELLOW OR BROWN."  The choices are: TOPAZ, TOURMALINE, PERIDOT OR TANZANITE.  It seems that more than one of these fits the color description but I'm not sure about the "oily" part.  Thank you for any help you can provide.

Answer
Hi Chris.
Are you sure these are your only options?
That oily part is getting to me too.
When I think of an oily luster, serpenting comes to mind (which does, by the way occur in all those colors).
OK, if I had to pick one of your options, I'd go with peridot as my first choice.  Sometimes it has been described as having an oily luster, although I think its closer to vitreous.
Hope this helps.
Feel free to write back if there is anything else I can do for you.
Barbra Voltaire, F.G.G.,G.G.