I don't see that anyone has yet mentioned it, but I've made GG's from diamond abrasives. Using diamond abrasives is way faster than any other abrasive, and not expensive. That was years ago, and I don't remember the supplier or the grit used.
I made a couple of 12x20 and several 4x5 glass plates by gluing the 4x5 plates to the bottom of an orbital sander and using them to grind the 12x20's.
Charley
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