Forehead crystal or jewel grants power to taker

Carbuncle

a brilliant gem embedded in its forehead

Amalthaea

In the usual telling one of the goat's horns is broken off, and it becomes the horn of plenty, filled with or producing an endless supply of whatever its possessor desires; some accounts have Zeus himself break it, others make the breaking accidental.

Uktena

On its forehead it bears a brilliant, blazing crystal called the Ulun'suti (or Ulunsuti), sometimes described as diamond-like or as glowing with its own light.