If an object is not made of glass it really isn't a glass. It is more of a plastic cup. Many bars offer plastic glasses to take outside when you sit in their beer gardens are go to smoke. Even those these are called glasses they are not really.
As far as anyone can tell... yes. There has not yet been found an example of glass that is not glass.I say nay, glass is melted sand, so glass is sand.
Technically, if they are made of plastic, then they aren't glasses. They might be fake glasses, for diguises etc... Often, real glasses do have a thin layer of plastic over the glass so that they...