Medieval stone cutters guild?

The "stone masons"

I think you mean the jewelers…aka the jews.

There are no hidden secrets in shaping a stone into a block. There is a vast ocean of secrets making a fortune in the gem industry. The whole thing is a racket. Price-fixing, collusion, artificial markets, fakes, flawed scoring. Rocks have very low intrinsic value. The entire jewelry industry is loosh.