Piezoelectric crystal grinding surface correction - has two diagonally opposite areas protected before etching remainder and lapping again

The incorrectly ground piezo electric crystal has its sloping faces corrected by applying to two diagonally opposite front and rear faces a protective layer of etching medium resistant material (13,14) and etching away the non protected parts of the faces (11). The flat rectangular crystal is then lapped between two lapping discs (15,16). This protective layer can be applied over each surface as a U-shaped layer, with the base of the U being much wider than the parallel arms of the U. In another form, the layer extends across half the top face, then down one edge, then underneath, across the bottom face. The top layer is much wider than the bottom layer.