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