toadstone

"Oh--and you need to drop a toadstone into the mixture."

Some various other names of the toadstone are:

It was also believed that there was a jewel inside a toad's head called a "toadstone".

By the Toadstone, I swear this swamp seems larger than the ambassador's yacht can possibly contain!"

New Oxford American Dictionary, under the entry "toadstone".

It was also believed that there was a jewel inside a toad's head, a "toadstone", that when worn as a necklace or ring would warn the wearer of attempts to poison them.

The toadstone (like the batrachite), also known as bufonite, is a mythical stone or gem thought to be found in, or produced by, a toad, and is supposed to be an antidote to poison.