Visually, there is a big difference between the 7mm Flanders and the 2.10 carat radiant. The radiant is a longer cut, and will measure 8.5 mm long, so it will be visually larger than the Flanders. You wouldn't see that much of a difference size wise between your friend's one carat round and the 7mm Flander's, so if you are looking for more visual impact go up to the 8mm.
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