pizza stone cleaning

Hi,
I have a pizza stone that I got a long time ago. It came with a small plastic scraper that I have since lost, but I think my problem goes beyond that: I've been storing it on my oven floor due to lack of cabinet space and I've been leaving it in there while I cook. I've noticed that lately it's become almost charcoal black on the top, although the bottom is still like new. Is this safe to cook on? Can I clean it? How? Or should I throw it away?

By the way, it is generally OK to store pizza stones on the oven floor?

Thanks,
Anne