A piece of clothing could be made of cotton, or polyester (nylon), or wool.
'made of' identifies the material/substance. The cotton and wool have been woven, but are still readily recognizable as natural fibres.
'made from' indicates that the original substance has been transformed into a new material. So,
it is hard to believe that glass is made from sand.
...that rubber, as in rubber tyres and rubber bands, is made from the sap of a tree.
- For Teachers