Almost all spectacle lenses produced now are made from plastic. Not only is this much safer than glass, they are also much lighter.
The draw back with a plastic lens is that it scratches much easier than glass and so care must be taken to clean them with a correct cloth and spray and ensure that they are stored properly in a case when not in use.
Hard coatings can be applied to plastic spectacle lenses to protect them. These do toughen the surface greatly, but do not make the lens entirely scratch proof.