Manufacturers make glasses from a huge range of materials. Most surprising of all, it is rare to make lenses from glass! While the first glasses were glass, opticians have since discovered more effective materials. These include plastic, or 'CR-39'. CR-39 is a plastic polymer. Polycarbonate is another material that is more impact resistant. It's typical of glasses made for kids, or accident prone adults (such as sportspeople). Trivex is a urethane based pre-polymer plastic. It's very similar to polyurethane. Both of those materials have 100% UV protection. High-index plastics (HIPs) are dense materials. These often contain sulfur. This element lets manufacturers use HIPs to make thinner (though heavier) lenses. All these synthetic materials don't mean glass isn't used altogether. It is still the material of choice for those with very strong prescriptions. Another benefit of glass is its higher scratch-resistance. Glass lenses are heavier, more delicate, and more difficult to produce though. Frames come in a variety of materials too. Here is a list of some of the metals manufacturers make glasses frames from. • Beta titanium (titanium + small amounts of aluminium and vanadium) • Memory metal (50% titanium, 50% nickel) • Beryllium • Stainless steel (iron + carbon + chromium) • Monel (nickel + copper) Glasses frames are also made from plastics. Some typical choices include Zylonite (or 'Zyl'), propionate, and nylon. Zyl is most common, and comes in a wide range of colours. Propionate is durable and common in sports frames. Nylon is rarely used nowadays, as it becomes brittle with age. There are also frames that come in a combination of materials.