The cut of a diamond is tricky terminology as the cut doesn't refer to the shape of the diamond ring. Morever, the diamond cut is in reference to the quality of the cut and it's ability to interact with or refact light.
According to the GIA, the following visual affects are studied to ascertain a diamond's interaction with light:
Brightness = Amount of internal/external light reflected from diamond
Fire = Scattering of white light into all colors of the rainbow
Scintillation = Amount of sparkle diamond produces, and it's pattern of white and dark areas caused by reflections
The GIA Diamond scale that guages the quality of the diamond cut is classified into five different categoritzations. Below are the five categories:
Round brilliant cut diamond rings are by far the most popular diamond ring shape. This diamond shape originated in the 17th century.
Ideal cut diamond rings represent approximately the top 3% of quality
Very good cut diamond rings represent approximately the top 15% of quality
Good cut diamond rings represent approximately the top 25% of quality
Fair cut diamond rings represent approximately the top 35% of quality
Poor cut diamond rings are the lowest quality and represent the bottom
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