The ASTM definition for a perpetual resource is "one that is virtually inexhaustible on a human time-scale". Examples given include solar energy, tidal energy, and wind energy,[26] to which should be added salt, stone, magnesium, diamonds, and other materials mentioned above. A study on the biogeophysical aspects of sustainability came up with a rule of prudent practice that a resource stock should last 700 years to achieve sustainability or become a perpetual resource, or for a worse case, 350 years.[27]