This is a chunk of a petrified mastodon tusk. My grandfather was working a deep-earth construction dig many years ago and his team came across a group of mastodon skeletons, all fairly intact. (I think this was Montana but I’m not sure.) After the discovery the crew boss called the Smithsonian but before the museum folks got there, some of the guys on my grandpa’s line broke up a tusk and each took a piece home to their families. This one sits in my bedroom by the window. It’s always been a good luck talisman.