There are two types of glass referred to in optics, which are crown glass and Flint glass. Crown glass is the clearer of the two and is most commonly used. However, glass in general is the clearest material of all the mediums. The index of crown glass is generally between 1.50 to 1.60. Where flint glass is the denser of the two ranging between 1.57 to 1.75. In multi-focal lenses (bifocal & trifocal) both crown and flint maybe used because of the different refraction scale. The most common composition of a glass multifocal lens includes having a segment usually made of flint glass to be completely immersed within the other glass, usually crown glass, giving a seamless surface and two different prescription ranges. Crown glass is the clearest material with an Abbe Value of 59. Theoretically speaking, the higher the index of refraction the lower the Abbe Value will be.
Another attractive characteristic of glass lenses is the scratch resistance and durability factor. Glass lenses are the most resistant to scratching of all the spectacle lens mediums. However, it is important to note that no spectacle lens material is scratch proof. Glass is also the most durable to strong solvents and aggressive solutions like acetone, paint thinner, etc. The biggest counter to the durability and clarity of glass is the weight of glass. Glass is the heaviest of all the spectacle lens materials by far and must be chemically or heat treated to pass FDA impact resistance standards. It is important to note that coatings for plastic and polymer lenses are getting more and more durable with technology.