A diamond cut is a style or design guide used when shaping a diamond for polishing and doesn't actually refer to the shape. Normally most consider the actual shape and appearance of the stone itself, rather than its ability to shine, but the Cut refers to the symmetry, proportioning and polish of a diamond and impacts its brilliance; this means if it is cut poorly, it will be less luminous.
We now know that nature is responsible for a diamond’s size, colour and clarity, yet the cut of a diamond is the work and artistry of an individual. When a diamond is cut precisely to the right proportion, light is reflected from facet to facet until it is bounced back outside the diamond through the top of the stone. For example, if two identical diamonds are placed side by side and one is less brilliant and fiery than the other, the fault lies in the cutting. A careful examination of the duller stone will show that its facet angles and proportions do not match those of a perfectly cut diamond and therefore cannot demand as high a price.