Of the 4C's, a diamond's cut has the greatest influence on its fire, sparkle and brilliance, making it, arguably, the most important C. Cut refers not only to the shape of a diamond, but more importantly to its proportions (how the diamond's angles and facets relate to one another), symmetry (the precision of its cut), and polish (the surface of the diamond).
A diamond's cut is harder to quantify than color, clarity and carat weight. The cut of a diamond is determined by the master craftsman, who formulates the best way to shape, facet and polish the diamond to maximize its beauty.
A diamond is essentially a prism of light, and diamond cutters work to let the most light shine through each stone. When done well, a diamond's cut can be the most important C.
When grading the cut of a diamond, laboratories evaluate the diamond for its:
The intensity of the white light that is reflected, both from the surface of the diamond as well as from the inside.
Also known as dispersion, it's how the light scatters through the diamond to create a rainbow of light, like a prism.
These are the flashes of white light that are visible when you move your diamond. These flashes are also known as "sparkle."
When a diamond is "Ideal-Cut," it means the angles and proportions of the diamond have been cut to produce the ultimate sparkle, fire and brilliance.