My favorite gem is ametrine! I think it is so cool that there can be two gemstones in one... Ametrine is usually cut in an emerald cut to show the color line, but it can also be cut in fancy ways so that it flashes with both purple and yellow... I love it...
I have a new favorite obscure gem every month I think... but lately I go weak in the knees for sphene... rainbows flickering from bright almost neon green.
My absolute favorite is chrome diopside! Nothing beats the rich deep green. Im not sure how rare it is, but whenever people ask me what it is and I say its chrome diopside, they go 'whaaat?' :)
Few people are aware that there are so many exquisite gemstones out there that are rarer than diamonds... Do you have an obscure gem that is special to you?