Forgive my ignorance, but isn't glass difficult to scratch? I always thought glass was only scratch-able with a diamond or glass cutter?
Nope. Glass is easy to scratch. Anything harder than glass can scratch it. There is a BIG MISCONCEPTION that people THINK the only thing that can scratch glass are diamonds. Not true. Diamonds can scratch and cut glass. There tons of other things that can scratch glass given the right angle, pressure and the like.
Stone
Metal
Which breaks down to keys, rings, dirt, sand, etc. Everyday things that are all around you can and will scratch glass. You don't need a diamond to do the dirty work.
Again, lots of misconceptions out there.