What are skulls for?

I found a Blue Skull! It looks special! Whats it for? Are there others? In what Acts can I find each of any of the others?

Do not tell me where the skulls are beyond the Acts they are located in please, I like a good treasure hunt.

Solution 1:

Skulls can be found throughout the game. For each skull, there is a corresponding altar. When you place the skull on the altar, you will unlock a special crafting recipe.

Skulls

Orange Skull: Act II.

Green Skull: Act I.

Purple Skull: Act II

Blue Skull: Act II

Brown Skull: Act I

Altars

Orange Skull Altar: Act II

Green Skull Altar: Act II

Purple Skull Altar Act II, during the quest Uninvited Guests.(Note: once the quest is completed, the Purple Altar becomes locked off.)

Blue Skull Altar: Act I

Brown Skull Altar