What is a Carat?

The term “carat” is a word that everyone knows is associated with the size gemstones and diamonds. It’s even one of the four “C’s” of diamonds. But, what is a carat?

The short answer is that a carat is a weight measurement not a dimensional measurement. But, there’s much more to the story that we are going to explore.

The Origin of the Word Carat

The word “carat” comes from the word carob. Historically, carob seeds were used by merchants as counterweights on balance scales. The carob seeds were particularly useful for determining the value of materials involved in the jewellery trade such as diamonds, gemstones and gold.

It’s important to note that the word “carat” is sometimes also used in place of “karat” when referring to gold purity. This can be confusing because karat is a fractional measurement of gold purity and is not a weight measurement. Carat weight measurement and karat gold purity are not the same thing. These terms are not interchangeable.

The Significance of Carob Seeds

Carob trees have a fruit that looks like a brown pea pod.  There is evidence of people using carob pods and seeds for various purposes going back to ancient Greece.

The seeds, also known as carats, have a fairly uniform weight of around 0.2 grams. Historically this relative uniformity was close enough to provide a reliable standard.  They were also easily available so carob seeds became the simplest method of weighing gemstones for trade.

Today, carob seeds are used primarily as an additive in food and as a substitute for chocolate flavouring. Although native to the Mediterranean region, carob trees can now be found all over the world.

How Carats Evolved

The tricky part is that, although everyone used carats as a weight measurement, everyone had a different interpretation of what constituted a carat. The measurement of carats was not standardized. Over the years some countries attempted to standardize the weight of carats within their own region but there were still differences between countries.

The Standardization of Carats

By the late 19th century the need for a standardized measurement became more and more obvious as global trade increased. In 1907, the Fourth General Conference on Weights and Measures established the metric carat as the official unit for measuring gemstone weight. The metric carat is defined as 0.2 grams, which is actually quite close to the average weight of a carob seed.

Finally there was uniformity in the gemstone industry. This standardization made trade much easier worldwide because there was a universally accepted measure.

What is a Carat Visually?

We use common carat size terms casually when we are actually trying to communicate a size. Terms such as a half carat, one carat, two carat, etc. That can make things quite complicated to understand. What does a one carat diamond look like? Why use a weight description when we are really trying to communicate a size description? Below is an image that will help you visualize typical carat weights in diamonds.

Since we know that carats are a weight measurement and not a size measurement, it makes sense that a carat will not translate to an exact dimensional measurement. The only thing that can be established is an average size of a half carat diamond, one carat diamond and so on.

Below is a diagram of the average sizes of four diamond carat weights. These average dimensions are only averages for diamonds. Average sizes correlated to carat weight is much more difficult to establish in other gemstones such as sapphire or emerald.


Carats and Gemstones Other than Diamond

For other gemstones such as sapphires, rubies, emeralds etc., there is no average size that we can correlate to carat weight. This is because other gemstones are usually cut to preserve carat weight. As a result, cutting is much less standardized in gemstones than it is in diamonds.

A gemstone cutter or lapidary may desire to preserve carat weight in gemstones for a number of reasons. If the gemstone is being sold by weight it is within their interest to preserve carat weight. Or, often coloured gemstones will be cut deep to preserve the saturation of colour.

Although there can be a generalized, average size for a one carat within a species of gemstone, the carat sizes can vary wildly within a species of gemstone and between species of gemstones.

So, Have we Answered the Question, What is a Carat?

For further information you can check out our diamond resource page about diamond carat. Or if you have a question that hasn’t been answered here you can you our complimentary, no pressure “ask a jeweller” service. We look forward to answering your questions!

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