Chewed Nut Shells
Alright - call me crazy (or obsessed) but I was out walking with the family the other day when we came to a park which had nut trees and squirrels. I noticed the chewed nut shells on the ground in large quantities and said to myself,"Self, those look like nice scale rocks or boulders." Self said,"Yes, and they are probably perfect for model terrain as they are extremely light and already come in a nice sand stone color too. You might also use them as the base material for urban fighting if you colored them a concrete gray." So I took a handful sample to bring back to show you all and I know where I can get more the next time I start looking for rock or debris for my table.
You just have to wash off the squirrel spit is all... pieces below are about half an inch at the longest.
Cheers