carats and gemstone gradingcarats and gemstone grading

Carats and Gemstone Grading


Carats are the units used to measure gem weight and one of the main factors in gem grading. Learn what affects the price per carat of various gem species.

3 Minute Read

What is a Carat?

In English, carat sounds the same as carrot, but we aren’t discussing vegetables. Carat also sounds the same as karat, but that’s a measure of the fineness of gold or the ratio of pure gold to alloy metals in a mixture. Karat is abbreviated as “K,” while carat is abbreviated as “ct.”

One carat equals 1/5th of a gram or 200 milligrams. You could also phrase that as five carats in a gram. A kilogram is 1,000 grams, or 5,000 carats. (My apologies to those of you on the metric system. To Americans, this sometimes needs to be explained).

Gem weights are occasionally expressed in common fractions like ½ or ¼ carats but are more commonly expressed as decimal fractions. (0.25 carats instead of ¼ carats, for example).

A carat is further divided into units called points. There are 100 points in a carat. So, for example, 25 points, 0.25 carats, and ¼ carats are all the same weight.

Carats, Gem Value, and the Law of Supply and Demand

To understand how carats affect a gemstone’s grade, you have to reflect on the law of supply and demand. Value is always a relationship between the demand…


Donald Clark, CSM IMG

The late Donald Clark, CSM founded the International Gem Society in 1998. Donald started in the gem and jewelry industry in 1976. He received his formal gemology training from the Gemological Institute of America (GIA) and the American Society of Gemcutters (ASG). The letters “CSM” after his name stood for Certified Supreme Master Gemcutter, a designation of Wykoff’s ASG which has often been referred to as the doctorate of gem cutting. The American Society of Gemcutters only had 54 people reach this level. Along with dozens of articles for leading trade magazines, Donald authored the book “Modern Faceting, the Easy Way.”

Related Articles

More Articles
Chrome Tourmaline Buying GuideChrome tourmaline</a>, shield cut by Loren Brown, 3.71 cts, Tanzania. © <a href=https://www.gemsociety.org/article/carats-karats-and-carrots/"https://rsagems.com/">RSA Gems</a>. Used with permission." src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" decoding="async" data-nimg="responsive" style="position:absolute;top:0;left:0;bottom:0;right:0;box-sizing:border-box;padding:0;border:none;margin:auto;display:block;width:0;height:0;min-width:100%;max-width:100%;min-height:100%;max-height:100%;object-fit:cover;object-position:50% 50%"/>

Chrome Tourmaline Buying Guide

Rare chrome tourmaline can show an emerald-like green color. With a highly desired color and tourmaline's excellent qualities as a...
Read More
A Consumer’s Guide to Gem Grading

A Consumer’s Guide to Gem Grading

Learn how gemologists evaluate color, clarity, cut, and carat for gem grading. Consumers can use this information to find the...
Read More
Clarity Grading for Colored Gemstones

Clarity Grading for Colored Gemstones

Don't underestimate the importance of clarity, one of the Four Cs of gemstone grading. For faceted gems, basic clarity grading...
Read More
Clarity Grading Diamonds

Clarity Grading Diamonds

Mastering clarity grading diamonds takes a lot of experience. This grading guide, with definitions, images, and examples, can help beginning...
Read More

Latest Articles

More Articles
Classic Engagement Ring StonesPastel purple sapphire</a>, 1.08 cts, 5.8 mm, round cut, Sri Lanka. © <a href=https://www.gemsociety.org/article/carats-karats-and-carrots/"https://www.thegemtrader.com/">The Gem Trader</a>. Used with permission." src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" decoding="async" data-nimg="responsive" style="position:absolute;top:0;left:0;bottom:0;right:0;box-sizing:border-box;padding:0;border:none;margin:auto;display:block;width:0;height:0;min-width:100%;max-width:100%;min-height:100%;max-height:100%;object-fit:cover;object-position:50% 50%"/>

Classic Engagement Ring Stones

Emerald, ruby, sapphire, and diamond are the most popular choices for classic engagement ring stones. Learn more about these traditional...
Read More
Broken Bangle — The Blunder-Besmirched History of Jade Nomenclature: Book Review

Broken Bangle — The Blunder-Besmirched History of Jade Nomenclature: Book Review

Is fei cui a better term for "pyroxene jade" than jadeite? Jeffery Bergman reviews Broken Bangle, a new book on...
Read More
Cuprite Value, Price, and Jewelry InformationJoel E. Arem</a>, PhD, FGA. Used with permission." src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" decoding="async" data-nimg="responsive" style="position:absolute;top:0;left:0;bottom:0;right:0;box-sizing:border-box;padding:0;border:none;margin:auto;display:block;width:0;height:0;min-width:100%;max-width:100%;min-height:100%;max-height:100%;object-fit:cover;object-position:50% 50%"/>

Cuprite Value, Price, and Jewelry Information

One of the rarest of all facetable gems, cut cuprites can show magnificent deep red color. However, these beautiful stones...
Read More
Gemstone Radiation TreatmentPhoto courtesy of <a href=https://www.gemsociety.org/article/carats-karats-and-carrots/"https://www.liveauctioneers.com/">liveauctioneers.com and <a href=https://www.gemsociety.org/article/carats-karats-and-carrots/"https://www.liveauctioneers.com/auctioneer/1081/eros-auctions-inc/">Eros Auctions, Inc</a>." src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" decoding="async" data-nimg="responsive" style="position:absolute;top:0;left:0;bottom:0;right:0;box-sizing:border-box;padding:0;border:none;margin:auto;display:block;width:0;height:0;min-width:100%;max-width:100%;min-height:100%;max-height:100%;object-fit:cover;object-position:50% 50%"/>

Gemstone Radiation Treatment

Gemstone radiation is a common but little understood technique for altering colors. Learn how it works and what gems usually...
Read More

Never Stop Learning

When you join the IGS community, you get trusted diamond & gemstone information when you need it.

Become a Member

Get Gemology Insights

Get started with the International Gem Society’s free guide to gemstone identification. Join our weekly newsletter & get a free copy of the Gem ID Checklist!