Lenses are generally made of either glass or plastic. Glass lenses offer the clearest vision and they’re also more solid and scratch resistance. Plastic lenses are typically made of either polyamide, polycarbonate, or a material dubbed CR-39. Polycarbonate is the lightest of the plastics and the most resistant alternative to crystal glass, while polyamide generally provides the best vision quality. CR-39 is the most widely used and the go-to material for polarized lenses.
Broadly speaking, most sunglasses are made of either metal, nylon fiber, or acetate. Nylon fiber aka plastic is one of the most resistant and versatile materials around. Generally speaking, metal is favored for its resistant properties and clean, classic look.
Acetate is a non-petroleum based plastic composed mainly of natural materials. As a material, it’s more supple than regular plastic, can be made totally transparent and can be used to make more complex colours.