The higher the sulpher content in the soil and air...the blacker the coin. If you sore your silver in an airtight container with a sachet of activated charcoal it wont tarnish.
LoL, Its not nature cleaning coins that makes silver come from the ground clean its the silver itself. Its a precious metal and dose not rust or corrode as easy as copper/clad. Gold is even harder to damage that way almost nothing can corrode it. You can dig a 2000 year old gold coin and it could come out shinny as new.