Mohs scale for metallic elements and alloys.
https://s3-us-west-1.amazonaws.com/h...s_fullsize.jpg
Previously:
https://www.homemadetools.net/forum/...538#post170195
https://www.homemadetools.net/forum/...250#post150146
https://www.homemadetools.net/forum/...938#post194729
https://www.homemadetools.net/forum/...480#post200805
https://www.homemadetools.net/forum/...293#post188541
