..at least they were in my quartzite tiles..
For those of you who don't know, they are a hollow tube with a diamond encrusted end.
I used a fairly low power cordless screwdriver to minimise chatter, started with a corner just dug in, and was able to make perfect holes through 10mm of stone with them.
Not fast, needed water and a lot of de clogging, but streets ahead of ANYTHING I have uses too make good holes in such tough materials.
Just posting this up for the record and for later google researchers.
Now if only teh ONE part of the shower glass design that I left to the glass company - the hinge slots for their hinges, had been as spot on accurate as the rest of the glass which I measured 6 times and drew twice..and fitted perfectly..:-(
Still they are making a new door, and the old one actually fits another shower that needs one..or will do, in due course..