The quality of the craftsmanship for the Finish and Proportions of a faceted diamond. A good make will have proportions that maximize Brilliance and Fire. A poor make will decrease sparkle and fire due to the loss of light as it travels through the stone.
Here is a graphic to demonstrate this for you. The light from the lamp at the top does not always come back out to your eye at the top, unless the cut is the right depth, for instance. Many other factors are just a important as basic proportions.