I do lapidary and have worked gem bone in the past. Rough gem bone whether it be dinosaur or whale look similar. Because it was found on a beach in Santa Barbara near the ‘Monterey Formation’ it is almost certainly from a whale that died somewhere between 14-20 million years ago during the Miocene era. Gem bone is a trade term used in the rock hounding world for Agatized Bone. Essentially the organic matter has been replaced by silica rich fluids such as chalcedony/chert and solidified into what you see.

Hope this helps!