Hardness of 7.5, round and pock marked, fractures exposing crystal lined pockets? Odds are you’ve got a piece of chalcedony on your hands. And while it’s not unheard of for it to be blue, it is rare and quite sought after by jewelry makers. As to what makes it blue, well I’m not certain. I know there are different mineral inclusions causing the color, but couldn’t tell you which you’ve got. In fact, the only included mineral I can remember that does show up is dumortierite. The only reason I remember this one is because I know someone who has a secret spot where he collects it. He claims this material doesn’t fade with exposure to the sun, unlike blue chalcedony with other inclusions.