The Leveridge gauge is a mechanical caliper-style instrument designed to estimate the carat weight of mounted diamonds without removing them from their settings. Developed in the early twentieth century by R. A. Leveridge, the device measures the diameter of a round brilliant or the length and width of a fancy shape, then references a paper chart that converts those millimetre readings to an approximate carat weight based on standard cutting proportions.

In a working bench environment the gauge remains useful for quick triage: appraisers, pawnbrokers and trade buyers reach for it when an unmounted weighing is impractical. Its accuracy is bounded, however, by the assumptions baked into the chart. A stone with a thick girdle, a deep pavilion or a steep crown will weigh more than the table predicts, and a shallow spread stone will weigh less. For this reason a Leveridge reading is treated as an estimate, not a determination, and any binding statement of weight relies on a balance reading after the stone is unset or, where unsetting is refused, on optical measurement combined with refined formulae.

Modern practice has largely supplanted the Leveridge gauge with digital callipers and proprietary mounted-stone weight estimators, but it is still encountered in older appraisal kits and remains a recognisable artefact of mid-century gemmological practice.