There were a bunch of them found with the Cheapside Hoard. They were frequently made into rings and other jewelry and worn as protection. Here's a pretty example.
The backs of these rings were open so the stone could be in contact with the skin of the wearer, to give them more potency. Queen Elizabeth I was known to wear one and Shakespeare wrote about them in 'As You Like It'-
Sweet are the uses of adversity;Which, like the toad, ugly and venomous,
Wears yet a precious jewel in his head.