Quick and Easy Buying Guide
Carat weight: 1 carat = 200 milligrams = 6.5 mm diameter. Doubling weight doesn't double diameter.
Diamond clarity: FL/IF/VVS/VS = super expensive, near perfect. SI = best value if you can check a photo for obvious inclusions (defects).
Color: D-G = colorless, expensive, only if you have money to burn. H-J = best value. Can go lower in gold metal settings than white metal.
Cut: Better cut ratings let more light into a diamond, making it sparkle more. Very important property, don't skimp here.
Set a budget and minimum cut (Premium). Go J color for gold and I/H for white metals. Go searching for SI1/SI2 clarity diamonds at James Allen. Pick a diamond with small/no inclusions. Choose a ring setting and buy it risk-free (60-day returns).
Actual Diamond Size of a 2.90 Carat Diamond
This 2.90 diamond will appear about the size shown in the photo, for a typical woman's fingers. An average round stone with a 2.90 carat weight has a diameter of 9.3 mm; bear in mind that this can vary slightly depending on the cut. This picture is scaled to show a size L 1/2 ring (British and Australia), which is equivalent to a US size 6 .
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Be careful of reading too much into just a picture - a picture alone won't be completely representative of all finger and hand types - you're best advised to get a bit of an idea in store by trying on actual rings with your partner. Don't forget to consider not just the overall size of your hand, but also if you have long and slim digits or squatter ones. Try and avoid making the mistake I made of examing a potential diamond engagement ring placed on my own, large male finger - remember that it is ending up on a generally smaller and more delicate female hand, which will make the diamond appear significantly larger.