Unit Conversions

The metric carat was standardised internationally in 1907 at exactly 200 mg (0.2 g). Before that date, the carat varied by country; the old English carat differed from the French, which differed from the Turkish, making gem weights in historical records difficult to compare. The 1907 standard fixed the unit absolutely, and today a 1.00 ct stone means the same weight in every country. In trade, the carat is subdivided into 100 points: a 0.50 ct stone is described as fifty points, a 0.35 ct stone as thirty-five points. The weight converter on this page handles carats, grams, milligrams, grains, and troy ounces, covering the full range of units encountered when reading historic inventory records, laboratory reports, and customs documentation.

Length conversion between millimetres and inches is a routine need when gem dimensions measured with a metric caliper must be communicated in markets that use imperial measurements, or when a published cutting diagram gives proportions in inches that must be scaled to millimetres before use in the carat estimator. The conversion is straightforward (1 inch = 25.4 mm exactly), but having a dedicated converter eliminates the mental arithmetic error that can occur when managing multiple measurements at once.

Temperature conversion matters in two specific gemmological contexts. First, the specific gravity calculation depends on the density of water at the temperature of the liquid used during hydrostatic weighing: water is densest at 4 °C (1.0000 g/cm³) and becomes measurably less dense as temperature rises, reaching 0.9957 g/cm³ at 30 °C. The SG calculator on the Measurement page applies this water-density correction automatically, and the temperature converter here lets you enter a Fahrenheit reading from a lab thermometer and obtain the Celsius value needed for that correction. Second, some refractometer reference tables and refractive index databases list temperature coefficients in Fahrenheit; converting to Celsius before applying a temperature correction to an RI reading ensures the adjustment is applied in the right direction and magnitude.