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).
Diamond Size on Hand of a 1.21 Carat Diamond
If you're wondering how big a 1.21 carat diamond appears when worn, here's a picture which provides a visual guide to this diamond's size. This 1.21 carat diamond (round shape) has a diameter of about 7.0 mm as shown in the picture.
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The picture shows a ring which is US size 6, which is equivalent to an L 1/2 size in places such as Britain and Australia. Your fingers could be significantly larger or smaller than the fingers shown in the photo, which are quite thin - keep in mind yours probably look quite different.