You've been killing dozens of these horrible slime beasts. After a lucky blow, a gleam caught your eye and you discovered each had a beautiful jewel surrounded by gold inside their slimy brains! So far you've collected 12 of these, though when you check your pack, you only count 9. Nothing to worry about through, you'll still make a hefty profit.
The Brain Jewel is worth 250 gold to an ignorant buyer.