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 Sizes on Hand of a 0.12 Carat Diamond
If you are wondering how large a diamond with a 0.12 carat weight appears when worn, here's a picture which provides a visual guide to this diamond's size. The woman's hand shown in the picture 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 present quite differently on your finger.
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The apparent size of a diamond is influenced by what it is shown against - her hand - a tiny hand will cause the diamond to appear significantly larger, with the reverse applying as well.