One method for determining the volume of irregular objects like rocks uses a technique called displacement. Archimedes (287-212 B.C.E.) is credited with discovering volume relationships. Here's how the story goes: Once there was a king who suspected that his crown might not be made of pure gold. He brought his problem to Archimedes, a "wise man" of the day. Archimedes pondered the question but didn't have an immediate solution. Later, as he was taking his bath, he noticed that the displacement of water in the tub was equal to the immersed part of his body. Archimedes leaped from the tub and ran naked through the streets, shouting, "Eureka!" His observation showed him how to solve the king's problem: Using the displacement-of-water method, he could easily calculate the volume of the crown. By comparing the weight of the crown to a lump of pure gold of the same volume, he found that the crown weighed less -- indeed it was not made of pure gold. As the king suspected, the crown was composed mostly of cheaper metals. Through measurement, Archimedes was able to expose the jeweler's dishonesty.
Before you begin measuring, estimate the volume of the rock. (Later, you can compare your estimate with the approximate volume you've measured.)
Take a graduated beaker marked in milliliters (or a measuring cup similarly marked) that is large enough to hold your rock. Fill the container halfway and record the water level.
Note that to measure the volume by displacement, you will need to fully submerge the rock in the water. Displacement will be equal to the amount of space taken up by the rock.