The first polyester fibers were produced by reacting ethylene glycol and either terephthalic acid or one of its esters to give poly(ethylene terephthalate). This polymer is still used to make thin films (Mylar) and textile fibers (Dacron and Fortrel).
This stamp also may be used for mailing items weighing up to 2-ounces. These Forever stamps will always be equal in value to the applicable price for the price category printed on them, at the time of use. The initial price and value for this stamp is 71 cents, effective May 31.
Short answer, currently 47 cents (US). Long answer, Forever stamps will always be worth whatever the current 1st class standard 1 ounce letter rate is. That rate may increase to 49 cents on Jan. 22nd. In which case, any Forever stamp would be worth 49 cents.