Solvent Ink. This stuff works! And nothing else does. We have tried it all.
We have a different item for the non metallic colors. They cost a bit less.
If you don't know which color to get go with Black. It is the most versatile. I strongly suggest that you get a bottle of StazOn cleaner also with these inks. I sell it in both spray bottle and also Dauber style.
Stazon inks are great for stamping on plastics, glass, metal and other nonporous surfaces (It works on porous surfaces as well).
See below for pictures of some of the Inks we tested out.