Buying Gemstones for Jewelry: Size to Weight Ratio

Loose Colored Gemstones

When it comes to buying colored gemstones for your jewelry settings, it is important to buy colored stones by size and not by weight. Gemstones are measured in metric carats, with one carat being equivalent to 0.2 grams. Gemstones are measured in weight to the nearest hundredth carat.

Diamonds are typically cut with standard proportions, which means you can purchase a 1 carat diamond by weight only and it will most likely fit your jewelry setting designed for that particular weight. Unlike diamond, colored gems are not cut to standard proportions. It is not possible to buy a setting designed specifically for a 1 carat sapphire by weight alone, because a 1 carat sapphire can range significantly in size depending on its cut.

Question: Why would similar shaped sapphires range in size if they are made up of the same material and weight?

Answer: Because colored gemstones are cut to maximize their most desirable trait - color. In most cases, colored stones are cut with heavier bottoms (pavilions) than most standard white diamonds. Since colored gemstone material can vary in color, lighter stones may be cut with heavier pavilions to deepen color, while darker materials may be cut with shallow bottoms to lighten color and reduce opacity. This is why jewelry settings for colored stones are always sold by size, rather than by weight.

Size-to-Weight-Ratio

Consumers often assume that a gemstone's size-to-weight ratio is the same for all gem types. However, this is not the case. Each gem type has a unique composition that results in a certain density and thus, size-to-weight ratio.

To help put this into perspective, 6.5 mm round sapphire, diamond and opal gemstones will have different weights, even though they have similar shapes and proportions. The image below shows some examples of the subtle size variations in different gemstone types that are cut with similar proportions and weigh 1 carat:

Size to Weight Ratio Chart for 1 Carat Gems - mm Scale

A number of gemstones have higher densities (specific gravity) than diamond, including many popular gemstones like garnet, ruby, sapphire, zircon and spinel. As a result, these gems will appear smaller in size when compared to diamonds of the same weight. This can sometimes disappoint customers who purchase based on weight alone, as the gemstones may turn out to be smaller than anticipated.

In conclusion, we suggest measuring the size of your setting and then selecting your colored stone - or, even better, first select the colored stone you love, and then custom build your jewelry setting to accommodate the gemstone!