First, let's not confuse diamond "cut" with "shape." Shape refers to the general outward appearance of the diamond, (such as round, emerald, or pear). When a diamond jeweler (or a diamond certificate) says "cut," that's a reference to the diamond's reflective qualities and not the shape.
A diamond's cut is the most critical of the 4Cs because it's what gives the diamond its brilliance, sparkle, and fire, the diamond's three main attributes. “Fire” describes the way the light disperses and how the diamond shows off the different colors of the spectrum. "Sparkle" also known as “scintillation” defines how much the diamond shimmers when you move it around in the light. When light enters the diamond, it is refracted and bounces back out in a rainbow of colors. If the diamond is too shallow and not cut right, some of the light will be lost out of the bottom. One that is cut too deeply will lose light out the side of its base. An Ideal Cut diamond will reflect most of the light through their table or top surface.
A diamond is comprised of five main components:
The largest facet of the diamond, which comprises the flat surface on the top of the stone, resembling a 'table'.
This is the top portion of the diamond, located above the girdle and extending below the table.
Forming the outer edge of the diamond, this is where the crown and the pavilion meet.
Located at the bottom of the diamond, the pavilion bridges the girdle and the culet.
The smallest facet of a diamond, the culet is located at the very bottom of the stone.
Ideal cut: Represents roughly the top 3% of diamond quality based on cut. Diamonds of this grade reflects nearly all the light that enters the diamond. An exquisite and rare cut diamond.
Very good cut: Represents roughly the top 15% of diamond quality based on cut. Diamonds of this grade reflects nearly as much light as the ideal cut, but for a lower price.
Good cut: Represents roughly the top 25% of diamond quality based on cut. Diamonds of this grade reflects most light that enters the diamond. It is less expensive compared to a very good cut diamond.
Fair cut: Represents roughly the top 35% of diamond quality based on cut. Still a quality diamond, but a fair cut graded diamond will not be as brilliant as compared to a good cut diamond.
Poor cut: Diamonds that are generally so deep, narrow and wide that they lose most of the light out the sides and bottom.