If it's petrified bone it might be a piece of dinosaur. I found a chunk of mammoth femur that was only the socket and maybe 8" of straight bone and it weighed 54 lbs. back in 1970. It was displayed in my local dive shop for years...sorry don't have a pic but it looked similar.
1st - I noticed this was your very first post - so, Welcome Aboard Thalia! You didn't list your state (or country) in your profile. So, you might consider jumping over to Sub-Forum: Select Your Area.... for information (i.e., clubs, hunts, finds, legends, maps, etc.) directly related to your state (or country).
2nd - I moved ya from HELP! over to WHAT IS IT? for more exposure.
If its so heavy why isn't it crushing what looks to be loose rolls of tape in your picture? Its leaning against a canvas bag frame and a plastic water cooler? Is this for real?