Green Stones.  The green stones are aggressive silicon carbide stones that love to eat hard metal.  THEY ARE NOT POLISHING STONES.  They have very specific purposes.

NEW SKIS.  See the index regarding tuning new skis. These stones are for working edges that have been hardened at the ski factory from misuse of the tuning equipment.
HARD SNOW. When you want your edges sharper than you are getting with the 180 grit ruby stones but you don't want to go to the extra steps involved with going to the files and subsequent polishing routines.  The finish we like for hard snow is sharpen with the green stones, polish with the ceramic stones, and that is it, this combination works great!