Fmulder directs us to an interesting piece of concept art, photographed (by SCLegacy.com) in the Diablo III section of the Blizzard Irvine museum.
Update: According to Kuangtu of Diablo3.cc, who saw this image last month during his visit, the caption for this piece read:
Black Soulstone Concept
What’s a black soulstone? Good question. One we probably won’t be able to answer until we’re playing Diablo III, later this… year?