Generally, it can mean facet design (as in step cut), shape (round, pear, marquise, etc.), or both (emerald cut, for example).  The most popular cut today is Round Brilliant followed by Princess but a diamond can be cut in any imaginary shape and form based on the shape of its original rough.  In another sense, cut means the proportions and finish of a diamond.  The term proportions refers to the relationships between size, shape, symmetry, weight distribution, and cutting angles in a stone, while finish refers to the details of facet shape, placement, and polish.

Under the GIA Diamond Cut Grading System, each diamond submitted is assigned 1 of 5 grades from EXCELLENT, VERY GOOD, GOOD, FAIR to POOR.  GIA does not offer IDEAL cut grade and neither does HRD.  Both HRD and IGI offer Hearts & Arrows grade to its highest standard cut grade diamond. IGI combines Ideal & Excellent in one grade.  AGSL assigns its top grade diamond to IDEAL (0).  EGL also has IDEAL for its highest cut grade standard.

Marcel Tolkowsky Ideal Cut Based on the legendary diamond cutter Marcel Tolkowsky Ideal Cut model, maximum light reflection through the crown can be obtained by following a set of meticulously calculated numbers.

 

Cut too deep - Lights enter the crown refract from one side of the pavilion to the opposite side then almost completely penetrated.  The diamond's brilliance is greatly diminished.	Cut too shallow - Lights enter the same spot bounce off only half its energy from the right pavilion.  When reaching the left pavilion further leak can be evident thus undermining overall brilliance.	Cut perfectly - Much of the lights enter the crown successfully refract from one side of the pavilion to another then reflect back to the other side of the crown.  Most of the energy retain thus maximum brilliance can be achieved.