Some years ago when I first got a LG U990 Viewty (which in some ways is the best phone I ever owned) I went swimming and left my phone in my bag. My phone happened to rest on my car keys and had vibration mode enabled, after a couple of missed calls I had a nasty scratched area on the phone screen. Since then I’ve been very wary about allowing metal objects to come in contact with a phone screen.
Now I have a Samsung Galaxy S with some sort of motherboard damage (it won’t even boot and I know it’s not a software issue because it was initially intermittent). A phone that old isn’t worth repairing (they sell on ebay for as little as $50) so it seemed worth testing how hard the screen is. The screen cover is Gorilla Glass which was the hardest glass available at the time the phone was new (apparently there are better versions of Gorilla Glass available now and my more recent phones should be tougher).
My first test was with one of my favorite Japanese kitchen knives, it didn’t scratch at all. Then I chose a knife sharpening stone as an obvious item that’s harder than a knife, it scratched the screen easily. A quartz pebble also scratched the screen when I used some force, so presumably concrete and brick would also scratch it. Tests with all current Australian coins and my car keys showed that the screen is too hard to be scratched by them. I also tested hitting the phone screen with my keys, I hit it much harder than would happen if I was to run while having my phone and my keys in the same pocket and there was no damage.
My conclusion is that any metal object you are likely to carry in your pocket is unlikely to cause any problem if knocked against the screen of a modern phone.