Glass

Glass has historically been the material of choice for ophthalmic lenses. Glass is most stable, scratch-resistant, and provides the best optical quality of all lens materials. However, since glass is more brittle than most materials, lenses made of glass must be tempered or heat-treated to give them more strength and make them safer to wear. Glass is available in a number of indices of refraction. As mentioned above, the higher the index, the thinner a lens will be for a given power. However, the specific gravity also increases dramatically with the index making high-index glass lenses much heavier. Chromatic aberration is also more pronounced in high-index glass.

Index: Crown 1.52; High index 1.60, 1.70, 1.80

Pros
Superior optics
Stable material
Scratch resistant

Cons
Does not accept tint
Not impact resistant
Heavy

CR-39

Developed by PPG during WWII, CR-39, also known as plastic or hard resin, serves as a much lighter lens material (approximately 50% lighter) than glass. CR-39, however, is far less scratch resistant and often must be coated to improve its scratch resistant characteristics.

Index: 1.498

Pros
Lighter than glass
Readily tintable
Less likely to fog

Cons
Susceptible to scratching (correctable by coating)
Lower index of refraction makes it less suitable for higher powered prescriptions