fix github issue 223 - rubbing ring on touchstone

fix github issue 223 - rubbing ring on touchstone

Fixes #223

The touchstone code treated all rings as if they had gemstones, but
quite a few don't and feedback could be unexpected.  Cited case was
an iron ring yielding a cyan (hi_metal) streak instead of the normal
iron result ("scritch, scritch").  A gold ring yielded a yellow
streak rather than a golden scratch.  I didn't test silver ring but
suspect it yielded a silver streak rather than a silvery scratch.

This changes touchstones to treat non-gemstone rings like other
classes of objects instead of like gems.  I made mineral rings keep
acting like gemstone rings--I'm not sure whether that's right.