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Buying Gemstones for Jewelry: Understanding Gem Sizes

For many people, gemstone sizes can be very confusing, not only for those who are unfamiliar with the metric system, but also for those who don't completely understand the exact format in which gemstones sizes are described and provided. The internationally recognized unit of measurement for gems and jewelry, including diamonds, colored stones and jewelry findings is always given in millimeters, usually rounded to the nearest two decimal points.

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

Inches in Fractions Inches in Decimals Millimeters Everyday Item of Approximate Size
1" 1.00" 25.4 mm Diameter of US quarter
7/8" 0.875" 22.225 mm Diameter of US nickel
3/4" 0.750" 19.05 mm Diameter of US penny
11/16" 0.6875" 17.4625 mm Diameter of US dime
5/8" 0.625" 15.875 mm Inside diameter of US ring size 5
9/16" 0.5625" 14.2875 mm Inside diameter of US ring size 3
7/16" 0.4375" 11.1125 mm Half the diameter of a US nickel
3/8" 0.375" 9.525 mm Thickness of an iPhone 4
5/16" 0.3125" 7.9375 mm Thickness of an iPhone 5
3/16" 0.1875" 4.7625 mm Thickness of a juice box straw
1/8" 0.125" 3.175 mm Thickness of 2 stacked US pennies
1/16" 0.0625" 1.5875 mm Thickness of a US penny
3/64" 0.0394" 1.00 mm Thickness of a CD
1/32" 0.03125" 0.79375 mm Thickness of a credit card

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.

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.

Gemstone size comparison chart
A 6.5 mm gemstone in comparison to standard USA ring sizes (inside diameter).
  • First Published: July-28-2014
  • Last Updated: August-23-2017
  • © 2005-2018 GemSelect.com all rights reserved.
    Reproduction (text or graphics) without the express written consent of GemSelect.com (SETT Company Ltd.) is strictly prohibited.
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Size and Weight

Gems are always measured in Millimeter (mm)

Dimensions are given as;
length x width x depth,
except for round stones which are;
diameter x depth

Select gems by size, not by weight!
Gem varieties vary in density, so carat weight is not a good indication of size

Note: 1ct = 0.2g

Size Comparison Chart