Jim in Anacortes
Life of the Party
Definitely an "Agate" and a very cool find. It is a form of cryptocrystalline quarts and I think they classify it as Chalcedony, but around here they just call them agates. I like to look for them. I cut and polished one very similar to yours that I made into a silver pendent. When properly polished, these stones can be astonishing.