I read somewhere that for geologic-scale signatures you need something that is: - Chemically inert - Recognizably artificial w/o context - Physically

I read somewhere that for geologic-scale signatures you need something that is: - Chemically inert - Recognizably artificial w/o context - Physically hardy - Planet-scale mass-produced On Earth, apparently "panes of glass" are one of the few things that qualifies.