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Diamonds are not all truly colorless, but it's the colorless, often called white, diamonds that other shades are judged against.
The Gemological Institute of America (GIA) has devised a set of guidelines to grade diamonds by color. The color of graded diamonds is compared to the color of control stones, preselected gems of a specific color.
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The Diamond Grading Procedure
Graded diamonds must be loose stones, because the color of mounted stones can be affected by their mountings. Stones are placed table-down, pavilion-up and viewed with a 10X loupe. A lettering system from D to Z is used to identify the amount of color present in each diamond, with D awarded only to the rare, totally colorless stone.
Diamond Color Grades
D-E-F
Colorless. For all practical purposes, diamonds graded from D through F are considered colorless. While subtle graduations from D through F do exist, and are measurable by a gemologist (using 10X magnification to map the stone's interior), to all but the trained eye these stones appear perfect.
G-H-I-J
Nearly colorless. Most diamonds sold are in the G through J range. While the untrained eye still sees these stones as colorless when mounted, they are graded as slightly tinted. "J" being more pronouncedly colored or tinted than a "G" color, most stores stock stones in this range, and order in higher grades as required. Diamonds graded G through I show virtually no color that is visible to the untrained eye.
K-L-M
Faintly tinted, usually yellow. Diamonds graded from K through M show visibly more marked color, and are classified as faintly tinted or colored.
N-O-P-Q-R
Lightly tinted, usually yellow. Tint can be seen with the naked eye.
S-T-U-V-W-X-Y-Z
Tinted, usually yellow, may progress to brownish. Tint visible to the naked eye, even when mounted.
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