Stones of fancy shape, such as square, or cushion shape, have to be formed in part by hand rubbing or "bruting" as it is called.
early 15c., "of or belonging to animals," from Middle French brut "coarse, brutal, raw, crude," from Latin brutus "heavy, dull, stupid," an Oscan word, from PIE root *gwere- "heavy" (see grave (adj.)). Before reaching English the meaning expanded to "of the lower animals." Used of human beings from 1530s.
1610s, from brute (adj.).