I seriously don't know what they put in these, but I've had these for years crawling around in ducts, on my knees on concrete floors, and doing auto/carpentry work and STILL haven't worn through the knees.  They don't seem to stain as easily either.  The "Durashield" actually lives up to its name and the jeans still look virtually new.  I have the indigo color and they have faded to a regular blue over the years, but no fraying anywhere.  My clip knife lives in the front right pocket and the leather patch is quite useful, all my other jeans have developed a frayed pocket there, not these.

Get some, they're awesome!