Water and pH are documented correlates of lichen and moss presence and dominance on rock surfaces, yet standard, defensible methods for their measurement are lacking. To address this deficiency we devised and tested methods for measuring absorption of liquid and atmospheric water, water retention and pH of rock surfaces. To adjust these measurements to cryptogam-relevant units we also developed an inexpensive method for determining rock surface area—by finding the increase in rock mass after applying a monolayer of homogenous sand grains with a known mass per unit surface area. We applied these techniques to compare volcanic (andesitic) and limestone rock surfaces from Yellowstone National Park, U.S.A. Andesitic rocks were less basic, absorbed more liquid and atmospheric water, and retained more water over time than limestone rocks.