answer:Someone once told me that if you have the sand and the ocean you never need to get pedicures. I have found this to be true. I used to run regularly on beaches barefoot (like maybe 2–5 mi distances), but only on sand that is pretty firm, and thus at least damp. Dry sand is too soft as is sand that is too saturated. I imagine that the coarser the grains of sand, the more likely it is to be “soft”, that is for your foot to sink and for running to be a real drag because smaller particles fit together more efficiently. Whether or not running on sand is good for you… I have heard that it is bad. It can be hard on your ankles. If you are not used to it, your legs can tire long before your lungs. I think in general the higher coefficient of friction a surface has, the more wear would be incurred on the sole of a running shoe all other factors being equal. Concrete would probably cause the most wear on soles since they often make grooves in slab surfaces to intentionally make the surface less smooth. Rock…there are all kinds of rock. A’a and Pahoehoe lava for example. Walk on pahoehoe, no problem. Walk on lava 30 minutes your brand new shoes have holes in the bottom. I guess that makes this particular type of lava worse than concrete by far. Pebbles I imagine to be smooth and clay I would guess is smoother. Clay is also probably softer than pebbles. I’m guesing clay causes the least wear. How do you like the Vibrams? I am thinking about investing in a pair.