Difference between organdy and organza

Both fabrics are transparent, crisp and woven with very fine, tightly twisted yarns in an open, plain weave. The difference between them is in the fiber content. Organdy is usually cotton or nylon, while organza can be made of silk, polyester or rayon. Organza is often used as a base fabric for embellishing. 

via 

http://sewing.about.com/library/sewnews/qa/aaqa0801b.htm