I think surface area is probably the more meaningful metric so while you're waiting for the water to clear, measure the volume of your rock using the Archimedes principle and then measure the surface area of said rock and let us all know the results. The method for measuring surface area is left as an exercise to the reader. Bonus points if you somehow use a laser. Extra bonus points if you use a blue laser.
http://www.thinkgeek.com/gadgets/lights/896a/