The Absorbency of Rock experiment is closely related to this one. Before soaking a rock in water, use an ohmmeter to measure the resistance from one side to the other. If you are using a piece of chalk, you could wrap some aluminum foil around the ends and touch the ohmmeter probes to the foil. With dry rock you should get very high resistance, probably too high to register any movement of the ohmmeter pointer. After soaking in water, the rock will probably have a lower, measurable resistance.
Salt water has a lower resistivity than fresh water.
In fact, pure water has a very high resistivity. But water is almost never found in pure form, particularly underground where minerals are dissolved in it. Send us your results. We'll periodically post new information here along with the reports from other people who have done these experiments. If you have a new question, ask an expert .