Carats, grams and kilograms: how rough is weighed and priced

A metric carat is 0.2 grams, so one gram is 5 carats and one kilogram is 5,000 carats. Cut stones are described in carats; rough appears in all three units. The same goods priced per carat, per gram or per kilogram give numbers that differ by a factor of five or five thousand, so check the unit first.

Where the metric carat came from

Before 1907 the carat varied from country to country, and none of the local versions had a simple relation to the gram. In October 1907 the fourth General Conference on Weights and Measures in Paris unanimously recommended that governments adopt a metric carat of 200 milligrams for diamonds, pearls and precious stones. The United States Bureau of Standards and customs service changed over on 1 July 1913. By then France, Germany, Japan and eleven other countries had adopted it. The old American carat had been close to 205.3 milligrams and was divided into fractions down to sixty-fourths; the new one is divided decimally, into hundredths.

Grams and kilograms for rough

GIA's 2017 report on Ethiopian emerald gave rough crystal weights in grams, one of 63.12 g, and the cut stones in carats. Mine output is counted in kilograms: Ethiopia's opal production for 2025/26 was reported as 6.4 tonnes. A 2016 World Bank study of the trade found rough opal bought from miners by the kilogram, while the best grade from Delanta could fetch up to ETB 3,000 per gram.

Large rough lots can still be priced by the carat. At an auction that closed in September 2026, Gemfields sold 3,047,650 carats of commercial-quality rough emerald, about 610 kg, in 44 lots, and reported the result as an average of USD 9.72 per carat.

USD 10 per carat is USD 50 per gram and USD 50,000 per kilogram. A parcel price covers the whole lot; divide it by the weight to compare it with offers quoted per unit.

Dry or wet weight for opal

The majority of Ethiopian opal is hydrophane: it absorbs water, and its weight changes with it. The laboratories of the Laboratory Manual Harmonisation Committee (LMHC) define hydrophane opal as opal that takes up enough liquid to affect its appearance and potentially its weight, and note that Ethiopian deposits produce an unusually high share of it. Their suggested test is to soak a stone for five minutes, dry it quickly with tissue and weigh it again; a change of more than 1% marks it as hydrophane. A weight that drifts over several days is a first sign.

The gain can be far larger than 1%. GIA's Bangkok laboratory soaked slices of an Ethiopian hydrophane opal and measured weight increases of 5 to 13%, and the slices took about a week to return to their original weight. A kilogram parcel behaving the same way could weigh 50 to 130 grams more straight from water than dry: 250 to 650 carats.

Scale tolerance

In the United States, NIST Handbook 44 names Class II as the typical accuracy class for gem weighing, and OIML R 76, used internationally, has nearly the same classes. A Class II scale in service may be off by one verification division (e) on loads up to 5,000 divisions, two up to 20,000 and three above that. On a scale with e of 0.1 g, a 1 kg parcel is 10,000 divisions, so a reading within 0.2 g, or 1 carat, is within tolerance. That is small beside the 50 to 130 g that water can add to a kilogram of hydrophane opal.

The division that counts is e, which on Class I and II scales can be ten times the last digit shown. NIST's weights and measures office warns that near a scale's minimum load the error becomes a large share of what is weighed, and recommends keeping most loads between a quarter and three-quarters of capacity. A single stone priced per carat belongs on a carat balance, not a parcel scale.

In a Maedot selection

Every selection gives weights and pricing. If you need to know when or how a parcel was weighed, ask us.

Sources

• BIPM, “Comptes rendus des séances de la quatrième Conférence générale des poids et mesures”, October 1907
• US Bureau of Standards, Circular No. 43, “The Metric Carat”, 1 November 1913
• Renfro et al., “A New Discovery of Emeralds from Ethiopia”, Gems & Gemology, Spring 2017
• Ashenafi Endale, The Reporter, “Regulatory Gaps Stifle Ethiopia’s Critical Minerals Industry Despite Record Revenue Figures”, 29 August 2026
• World Bank and Levin Sources, “The Commercial Potential of Ethiopia’s Coloured Gemstone Industry”, 2016
• Gemfields, “Auction Results: Commercial-Quality Emeralds”, 11 September 2026
• Vertriest et al., “Land of Origins: A Gemological Expedition to Ethiopia”, Gems & Gemology, Spring 2019
• Laboratory Manual Harmonisation Committee, Information Sheet 13, “Hydrophane Opal”, version 3, May 2025
• Suthiyuth and Weeramonkhonlert, “Hydrophane Opal Treatment”, Gems & Gemology Lab Notes, Spring 2016
• NIST, Handbook 44, Section 2.20, “Scales”, 2026 edition
• Rick Harshman, NIST Office of Weights and Measures, “Making Sense of the Min Marking on Class I and Class II Scales”, revised 15 April 2020