It looks like a river-worn quartz pebble.

It looks modern, because it looks mostly made of bone, with little of no mineralization.  If it was from a fossil fish, you might expect it to have more of the bone replaced with rock minerals like silicates.

I agree with the bullfish predentary bone from a modern fish. It is definitely NOT from a sea urchin. That must’ve been a good size fish!

I think you need to post a picture with a little more detail, especially of the head region in order to attempt an ID.  Did the specimen come with a note telling where it was from by any chance?