I have a rock that I inherited from my grandmother who collected strange rocks. The story goes that a friend or relative gave it to her after they discovered a

I have a rock that I inherited from my grandmother who collected strange rocks. The story goes that a friend or relative gave it to her after they discovered a hole in their barn roof and found this rock embedded in the ground inside. This was sometime in the 1920s or 1930s. She always called it her 'meteorite'. It is about the size of a smashed tennis ball. It is heavy, completely black and has a strange pattern of lines and dots covering it. I am a bit skeptical that it is a real meteorite. A magnet won't stick to it and the pattern doesn't seem like something that a rock falling through the atmosphere would develop naturally. It is certainly a strange rock and I have never seen anything else like it. Anyone know a good way to determine if it is legit?