Stone Gauge Baker Style for Making Jewelry - SFC Tools - 35-558 Holes in this gauge are used to measure the sizes of stones in both carats and millimeters. Extensions are used to measure the sizes of mountings, also in both carats and millimeters. The carat gauge can measure 19 sizes from 1/32 to 4 carat stones in quarter intervals. The millimeter gauge can measure 19 sizes from 2 to 11 millimeters in half-millimeter intervals. Gauge measures 4x4 (10x10 cm) and is made of aluminum.