Piece of Mt. Vesuvius in my tank?

Give the vinegar test. Bubbles mean it will affect your chemistry, no bubbles means it might not. Best way to check is to put it in a bucket with some water from your tank, let it sit a week, and then test the pH, GH and KH (assuming that you test these value before adding the rock as well! ). If there's a huge change, I wouldn't add it.

You might also want to consider heavy metals. A lot of volcanic rock contains arsenic and other nasty elements. No way I can think of to test, without spending way too much money. For a FW tank, I'd toss the stuff in if it passes the fizz test, but keep it in mind if your fish start acting funny.