The fabric of washed silk is not polyester fiber, but mulberry silk. Mulberry silk is a type of silk. Silk threads are extracted from the cocoons made by silkworms eating mulberry leaves on artificial or wild mulberry trees, and are twisted and woven to form silk quilts, silk socks, silk clothes and other silk products. Polyester fiber is a synthetic fiber made from polyethylene terephthalate by polycondensation of organic dibasic acid and glycol, and then spun.