Bismuth telluride is a compound of bismuth and tellurium. This would seem pedantic were it not for the fact that tellurium is rare as gold.

There's another tellurium-based pipe dream in the form of cadmium telluride solar cells, which have the lucky coincidence of being highly efficient and very cheap to manufacture. Unfortunately... rare as gold.

http://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth%... -- tellurium variously at 0.005, 0.001, and 0.001 ppm, and gold at 0.0011, 0.0031, and 0.004 ppm. So "similarly rare."

Friends don't let friends try to save the world with tellurium.