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The cut of a diamond is related to its proportion and finish. It affects the overall appearance and gives
the diamond its sparkle. In January 1, 2006, GIA instituted a
diamond cut grading system for Round Brilliant diamonds. This system
is evaluates the loose diamond's overall appearance by using computer
modeling and other methods to predict the levels of brightness, fire, and
scintillation. Features such as weight ratio, durability, polish and
symmetry are all included it the GIA cut grade. GIA cut is graded on
a scale from Poor to Excellent. #1 Diamond Source sells GIA
graded loose diamonds with "Good" or better cut grades.
A diamond gets its brilliance and scintillation by the cutting and
polishing of its facets, allowing the maximum amount of light that enters
through its top to be reflected and dispersed back through the top.
With proper cutting (see diagram below) the light passes through the top,
bounces off the sides, and then travels back out the top, giving the
diamond optimum brilliance. If the diamond is cut too deep, light passes
through the side of the diamond. If the diamond is cut too shallow,
light passes through the bottom of the diamond, also inhibiting maximum
brilliance.
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