Diamond cut grading according to GIA:
Diamond's "cut" encompasses shape, pattern, proportion, symmetry, as well as position, and style of the diamond facets. There are two essential types of diamond facets: step- and brilliant-style.
Most people are familiar with kite- and triangular-shaped brilliant facets. The majority of modern diamonds such as rounds, ovals, princesses are brilliant-cut.
Step-cut facets are rectangular or trapezoid in shape. Emerald cuts, Asscher-cuts, and baguettes are all step-cut diamonds. Rarely diamonds combine both step- and brilliant-cut facets on the same stone. Such stones have "Mixed" cut.
The cut significantly affects a diamond's optical properties; it determines how well the stone reflects and breaks the light. Round brilliants with excellent cut, symmetry, and polish grade are called "ideal cut diamonds."
Sometimes these diamonds are called "Triple X," "XXX," "ExExEX". Some ideal cut diamonds with a specific pattern of facets can be classified as "Hearts and Arrows" or "Hearts & Arrows." The idea that an ideal cut has a superior look is preposterous.
Some people find a diamond with a small degree of asymmetry to be more attractive. They argue that a diamond with perfect symmetry lacks the randomness in reflections that make it come alive.
A precursor of the ideal cut, the "American Standard" cut was developed in 1919 by a Polish engineer Marcel Tolkowsky.
His formula was perfected over the years, eventually resulting in the ideal diamond cut. The cut grades are "poor," "fair," "good," "very good," and "excellent."
A property called "sparkle" does not exist. This word that radiates the lack of maturity probably refers to both brilliance and dispersion. The brilliance is a repeated blinking of diamond facets in random order. At the same time, dispersion is the ability to scatter and polarise light, causing the rainbow effect commonly called "fire" in diamonds.
Some diamonds are cut to maximize brilliance, others for fire. Brilliant cut maximizes brilliance at dispersion's expense. Antique and step-cut diamonds popular when ballrooms were still lit with candles are cut to maximize the fire.