This loupe can be used to estimate the weight of a mounted gemstone or to measure the table, or facets, etc of the stone. The distance between measuring scale and the lens is adjustable so you can obtain a sharp image of the measuring scale imposed on the gemstone. The measuring scale ranges from 0 to +/-15mm, in 0.1mm increments, so that gives you a full 30mm scale.
WARNING: California's Proposition 65