I've never cared much what materials clothes are made out of, as long as they look and feel good, and seem like they'll last. Sadly, I've proved to be a poor judge of the last point.
I don't wear jeans, though. I honestly have no clue what the pants I am wearing now are made of
EDIT: A quick check reveals they are 100% cotton. Now I know.
And knowing is half the battle!!