[barrkel]

Having it slide across an otherwise flat surface with indeterminate grit on it usually does the trick.

Aluminium oxide, for example - it comes off as white powder on motorcycle engines - and is pretty hard stuff, the same stuff as sapphire and ruby. That can end up on surfaces via motorcycle gloves if you ride motorbikes, if not from your own bike, then grit thrown up by tyres on a wet day.

I mean, phones don't scratch easily, but they do acquire a tiny scratch or two every few months, and a bad scratch maybe once or twice a year.