Agate comes in a variety of colours and patterns. Although agates may be found in various kinds of rock, they are associated with volcanic rocks but are also found in certain metamorphic rocks. It's one of the most common materials used in the art of hardstone carving having been first discovered in the Dirillo river in Sicily over 3,000 years ago.
These are quite hard stones. If you have a mortar and pestle at home there's a good chance it's made from agate. There is such variety in the swirling patters we have with these agate ankers that they really pop on a shirt.