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 2.16 Carat Diamond Size on Hand
This 2.16 carat diamond (round shape) has a diameter of about 8.5 mm which you can see in the image. The woman's hand pictured has long and thin fingers - your fingers may or may not be similar - if they're not, be aware that a diamond engagement ring may look quite different when worn. The visual size of the diamond and ring can be affected by whether you have thin or thick fingers, as well as by the length of your fingers.
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Look at the difference between the size of the woman's hand and the man's hand in the photo - you can see that the woman's hand is much smaller and more petite (which is typical), so the diamond appears larger on her hand than it would appear to be on his.