A diamond's cut is the most important characteristic. While nature determines a diamond's carat weight, clarity and color, it's the hand of the diamond cutter that releases the stone's fire, sparkle and beauty.
In the industry, diamantaires refer to "make" when they speak of cut and proportion, i.e. excellent, fine, top, good. A "well proportioned" diamond refracts light from one facet to another, creating a shimmering brilliance. Diamonds that are cut "too deep" or "too shallow" refract light out the bottom, causing the diamond to lack brilliance.
So, when shopping for diamonds, pay particular attention to cut because with all things equal, a greater carat weight doesn't necessarily mean you're getting a better value. The cut of the diamond is the determining factor.