Stonewashed jeans are, in fact, blue jeans that have been washed with stones. Really, I'm not kidding.
In the begining, to soften the fabric and to give it a worn look, the jeans, after they were assembled, were given a wash with pumice stones and fine volcanic gravel. The pumice was strong enough to abraise the cotton but not sharp or hard enough to do serious damage. Whole economies of small, pacific volcanic islands were built on the trade with denim companies of pumice stones. Now-a-days. however, stonewashing is done with enzyme washes, sand, and ceramic balls give the jeans their worn look.
Stonewashed jeans fell out of style in the early 90's along with the fall of the hair bands, but are now making a resurgence. The jeans, not the hair bands, thankfully.