The most crucial factor in determining the beauty of a diamond is cut (not to be confused with shape). If a diamond is not cut with careful precision, it will appear lifeless. Diamonds that are cut for beauty, not size, will maximize brilliance (intensity of light reflection), sparkle (the flash of light when the diamond or light source is moved), and fire (dispersion of light into spectral colours).
From a value perspective, cut is measured by its proportions (correlation between the table, crown, and pavilion), symmetry (size and accuracy of individual facets), and polish (condition of a diamond's surface). These characteristics are evaluated and graded based on their impact on the beauty of a diamond.
How Pavilion Depth Affects Cut:
When the depth of a pavilion is too shallow the girdle of the diamond will reflect through the table, causing a "hallow" effect. If the pavilion is too deep, the diamond will appear dull and dark.
Joseph-Anthony diamonds are always cut to maximize light performance rather than size.