Grading colored gemstones
Color stone grading is considerably different from diamond grading
While the “4 Cs” of diamond grading still applyColorCutCarat weightClarity
By far the most important is Color
Grading a colored gemstone
Some stones just “seem” better than others. Let’s see why.
Three main factors merge to create the best colored stones:Good colorGood shape appealGood light return
We can quickly estimate these to find the best stones
Color “Color” is the most
important component of value in colored stones
Use modifiers to note the tone and saturation of the stone
Then note the hue of the stone. Stones rarely are pure colors
Color-tone Tone is the relative
lightness or darkness of a hue Extremely light Very light Light Medium light Medium Medium dark Dark Very dark Extremely dark Black
Color-saturation Saturation is the
strength or purity of a hue Grayish (brownish) Slightly grayish Very slightly grayish Moderately strong Strong Vivid
Color-hue Hue is the shade,
tint or sensation of a color Most stones aren’t
purely one color, but modified shades of two colors
Orangish-redReddish-purple
Light Properties The light properties
of the stone are next in importance in determining value. We judge the quality based on three factors Windowing Light return Extinction
Windowing “Windows” are
areas of stone transparency
They are visible as light areas on the face of the stone through which you can see objects beyond
Windowing Wave an object, like
a pen or pencil, underneath the stone while looking through it and estimate the overall surface percentage that shows motion in it
Light return and extinction
Light return is shown by bright areas on the stone’s surface
Tilt the stone up to 30° in each direction and estimate the percentage of the surface that is light
Good stones = +45% light return
Light return and extinction
The dark areas represent light that is lost from the crown of the stone. You estimate its percentage also.
Light return and absorption
What are the window, light, and extinction percentages of this stone? Window: about 0-5% Light: about 45-55% Dark: about 45-50%
Light return and absorption
Be careful with reflections on facets. You need to “look through the reflection to see the internal light and dark areas.
Let’s completely grade a stone Stone ID
Rhodolite garnet
Stone shape Pear, broad wings
Stone style Faceted
Color Medium, moderately
strong, purplish red