This translates to Green-Blue Hue, Medium Dark Tone, and Moderately Strong Saturation. A gem's basic body color. A normal person with average vision can see up to 150 distinct hues, but in we use 31. This permits us to consistently, and in simple terms, communicate a stones hue. Click here to view the GIA Gem Set Hue Wheel.
Tone is a color's lightness or darkness.
You can get an idea of what tone means in color if you consider that many pinks are light tones of red. For transparent gems, you normally deal with only levels 2 through 8. Each tone level has a number and a corresponding word term. The GIA Color Grading System divides saturation variations into six levels that run from 1, grayish or brownish, through 6, vivid. You can see the number and word term associated with each saturation level in the photo of the Gem Set Saturation Samples.