Okay folks, let's break this **** down.
Concern over scratched screens is a holdover from the days when phone screens were made of plastic. Plastic, being extremely soft, is prone to scratches by all manner of ordinary items: keys, money clips, coins, scissors.
Modern smartphones generally have glass screens. Glass is extremely hard. While even items we consider soft can shatter glass - don't throw a phone book at your living room window - only very hard things can scratch it. Most rocks, sand, and jewels can scratch glass. I do not know of a single metal which can scratch glass (thought some may exist).
This is why screen protectors are generally not needed for glass screens: Anything which can scratch your screen - such as a diamond ring - will typically tear through that screen protector with ease.