Describing Gems in Millimeters: Some Comparisons to Help

When buying gems or jewelry online, it's hard to know what "8mm" means in terms of size.  You can look at a ruler and on the opposite edge of Inches are CM (centimeters) and MM (millimeters).  Inches have lines for each 1/8 of an inch and 1/16 of an inch.  On the centimeter side, there are 10mm to one cm, and about 25mm to one inch.  But that's still hard to judge the size of a gemstone. Chain widths are also measured in millimeters, so it can be surprising to see how tiny a chain will look in person. Chains can be as small as under 1mm (.8 or even smaller).  Nylon or silk cords can be less than .5mm---like a thread.

Rulers have inches on one edge, mm on the other

Sellers take very close pictures of the jewelry they're selling and buyers can be disappointed at how tiny gems actually are, even with measurements listed.

1-ct diamond ring = 6.5mm diameter

So I put together a quick chart to help judge sizes, comparing millimeters to everyday objects and the approximate carats of round-cut diamonds:

                         SIZE              DIAMETER

• 1mm          Grain of sugar      
• 1.25mm     1-point round diamond
• 4mm          .25-ct. diamond, match head
• 5mm          Pea, and .50-ct. diamond
• 6mm          Pencil eraser 
• 6.5mm       1-carat round diamond
• 8mm          2-carat round diamond 
• 9.25mm     3-carat round diamond
• 10mm        Standard thumbtack head
• 11mm        AAA battery, and 5-ct. diamond
• 14mm        AA battery
• 16mm        Button on a pair of jeans
• 17mm        A battery
• 18mm        Dime
• 19mm        Penny
• 21mm        Nickel
• 24mm        Quarter 

•  3mm  = 1/8"
•  6mm  = 1/4"
•  8mm  = 5/16"
• 10mm = 3/8"
• 13mm = 1/2"
• 19mm = 3/4"
• 23mm = 7/8"
• 20mm = 1"

 I hope this helps you "see" the sizes of gemstones so you know exactly what you're buying!