I don't know what kind of sand they are using, but as lond as you are not rubbing it on the surface like sandpaper it won't harm the stone. It may plug it up and have to be picked out, but the worst it could do is make the stone look dirty. If you want a similar method, less messy, and yes environmentally safe, use corn starch and food coloring. Take water and corn starch to make a paste, spreadable thick, not runny or too thick. About like peanut butter. Give it some food coloring that is contrasting to the stone - like black on white, red on green, something that stands out. Mix up the paste and using something like a plastic putty knife, spread it on the surface just enough to see the faded images. It may take a few passes with light layers to get it just right. Try to get it only in the letters and clear the surface. Take a photo and it may be easy to fix on a computer to dress it up so you can't tell it has been done. Of course, please use a paint brush and some water to clean off the paste when you are done.