My father around 20 years ago came across what is a square like rock,black looking with what looks like silver underneath if you rub it. Its about 6 to 8 inch square and weighs about 20 to 30 lbs im guessing he had a friend of his have it tested years ago and it came back as unknown,this thing is heavy as can be but you can take pieces off of it,you rub it and it turns like a silver color then after a while back to black, any idea what it could be?