The more recent smartphones have screen with scratch-resistant glass, but every once in a while you'll see that your phone has picked up another fine scratch. XDA Developers nails down the source: it's not the change or the keys in your pocket, it's the sand.
XDA did a bit of experimentation with a number of Spigen screen protectors, which use tempered glass to keep scratches off your phone (not unlike the "Gorilla Glass" that coats most modern smartphones). They tested it against keys, coins, sand and other materials to see what scratches it the most.
Surprisingly, most materials didn't leave a mark, but that hard, gritty sand that's floating around in your pocket actually has the potential to leave scratches on your phone, showing us that screen protectors can still be useful. Check out the video above to see their tests in action (although close your eyes at the end, it gets pretty scary when she does the drop tests).
Truth About Gorilla Glass -- Drops, Tests, Composition [XDA Developers]