Rush seats are typically woven using either fiber rush or natural seagrass rush.  (In times past they were often woven of twisted plant materials (such as cat tails and other "rushes"), which was a far more time-consuming job, having to be hand-twisted as the seat was woven.)

Fiber rush is an incredibly durable twisted paper product that has withstood the test of time!  According to the website for Danish furniture manufacturer, Carl Hansen & Sons, "paper cord" was first developed in Sweden during the war, (I'm guessing WWII), for use in grain binders as a substitute for jute.  Their brochure says it lasts up to 50 years before needing replacement.  I have seen a number of seats that were easily that old, and largely in good shape if they were taken care of, (ie, not left out in the rain, or ruined by cats, kid's spills or scissors, etc.!

(For photos of chairs woven with Danish Cord, which is the Danish version of fiber rush, check out that section on the menu.)

Natural seagrass rush is twisted seagrass, and while it is lovely, and when it's fresh has a smell that reminds me of sweet grass, it gives a more "textured" seat due to the variability in thickness of the material.