Old Navajo Bolo - signed/dated - mystery stone (petrified wood?)

I purchased this Navajo bolo years ago, as I recall as part of a lot. Sorry for the quality of the photos - I chose not to clean the silver, at least for the moment. It is signed with three initials - I believe A M O - and dated 3-75, at least I believe it is a date. My question regards the stone - is anyone able to identify? I'm thinking it might be petrified wood? Welcome any/all info/comments.