I agree with Alistair. It looks like sandstone. I've used similar in tanks in the past with no trouble at all, although it's not top of the list of ideal stones as it can break down and reveal lumps of other stuff. Generally though the harder it is the better for fishkeeping purposes. So no guarantees but is probably fine.
Oh, and for the record, the general advice is to not boil rocks. Pour boiling water over them by all means, but don't boil them, once in a while they'll explode and that can make a mess of the kitchen, and sometimes the person doing the boiling.