Question

what is the reason behind the brightness of the well cut diamond​

Answer 1

In fact, a well cut diamond will reflect more light and appear larger than it actually is. It can also make the face-up color look brighter, and at the same time, mask or camouflage inclusions. This is because a well cut diamond has greater brilliance, fire, and scintillation that can easily conceal flaws

Answer 2

it is the structure of diamond which reflect the light to give brightness . the structure of diamond is such that it reflect most of the light falling on it which give it a bright appearance