Plastic CR-39 Lenses are the most basic and affordable lens material on the market. Prescription plastic lenses are almost half the weight of traditional glass material, is significantly more resistant to shattering, and are still somewhat resistant to scratches. (Note - adding an anti-scratch coating to CR39 lenses improves this feature.)
Plastic lenses are easier to to die, if you are looking for a unique color sunglass lens. If your prescription is less than -2 diopters, you can save a considerable amount of money by selecting Plastic CR39 lenses as opposed to more expensive high index lenses, which are not necessary at this prescription.
Click here to read more about other types of lens materials (for example, plastic CR39, polycarbonate, and high index lens materials).