If you bought the Collector's Edition of Diablo you had the opportunity of getting the Mini Diablo as a non-combat pet. Further, in the patch notes for Patch 1.10 this tiny little note showed up:
"The Diablo Stone now has a unique icon."
Previously it looked like the Citrine gem icon.
Frankly, although the new icon is unique, I rather liked the old one. It reminded me of the very end of Diablo I. (I had just played and finally conquered this entire game to heroically save this SoulStone from the forces of evil, only to be rewarded with an animation where I SHOVE IT FULL FORCE INTO MY FOREHEAD! (more info here.))
Anyway, my trauma aside, Mini Diablo does two interesting things. He periodically will spout fire. I was starting this new character and talking to a quest giver and I look over and see this:
Don't mind me, Mini-D. I'm just negotiating a way to pay for our next meal.