Buying Gemstones for Jewelry: Understanding Gemstone Sizes

Size of gemstones can be quite confusing for many individuals, especially for those unfamiliar with the metric system or those who may not fully grasp how these sizes are specified. The standard unit of measurement for gems and jewelry—covering diamonds, colored stones, and various jewelry components—is consistently expressed in millimeters, typically rounded to two decimal places.

For our US customers, the following inch to millimeter conversion table may be helpful when buying gems by size for your jewelry needs:

The format used to describe gemstone sizes can also vary depending on the shape of the specific stone. For round gemstones, usually two dimensions are detailed; diameter and depth. For ovals and fancy shapes, gemstones sizes are described using three dimensions; length, width and depth.

Gem buyers should also be aware that gemstones are rarely cut with perfect symmetry, and depending on where a stone is actually measured from, its size may be given as differing values. Thus, it's highly recommended to allow for some variance in regard to exact millimeter sizes when buying colored gemstones for your jewelry settings. Explore our diverse selection of gemstones for sale.

How Big Will My Gemstone Look?

The picture below may be helpful when deciding how big or how small a gemstone you require for your jewelry. This image shows an outline of how a 6.5 mm round stone may look when set in standard ring sizes. A 6.5 mm stone may look large in delicate jewelry designs, but in chunkier designs such as a men's rings, a 6.5 mm stone will appear much smaller than you might expect.

A 6.5 mm gemstone in comparison to standard USA ring sizes (inside diameter).