Not all glass tile is the same
A quote from a posting on the JB Tile Forum:
"Not all glass tile is the same. Some scratch and some don't. Your idea to test it on some scrap is a good idea."
While we have had great acceptance, use and results with SpectraLOCK with many types of glass tiles on many installations, there are many different types of glass tiles available.
It is best to test the specific grout that you plan on using BEFORE making the final grouting installation...!
This is true for SpectraLOCK as well as "normal" Portland Cement based grouts.