Not a technically separate stone, agate is a combination of materials.  Formed by layer upon layer of material, agate is frequently banded with bold lines and is one of the most striking semi-precious stones for that reason.

Associated chiefly with the mind and storms, agate is a soothing stone.  It's said to be able to cure and protect against poisoned bites such as snakes and scorpions.  Frequently associated with Persian magicians, agate supposedly aided them in their control and calming of storms.  Wear agate to improve your focus, memory, and eloquence.