However, this in itself is not a cause for worry. The stone on a 3-carat gold diamond ring, for instance, can seem to go dark for a simple reason: it reflects too much light for the eye to take in. That causes the pupils to contract, just the way they would if you stared at the sun for a couple seconds. This results in the foreground and background appearing much darker than actual. Simply stepping back and turning your head a bit to the side would reveal the stone’s true brilliance; and if you have a GIA certificate, there is even less reason to worry that you may have been swindled.
Since it is your eyes that have a problem, sit in the shade and relax them awhile. Now look at the diamond anew, and you will no doubt see it is worth every penny you paid for it.