Grading Colored Gemstones Carl Wozniak

Grading colored gemstones  Color stone grading is considerably different from diamond grading  While the “4 Cs” of diamond grading still apply  Color  Cut  Carat weight  Clarity  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 color  Good shape appeal  Good light return  We can quickly estimate these to find the best stones

Which would you buy? $2, $45.00

Stone shapes

Stone style  Stones can be  Rough  Tumbled  Faceted  Cabochon  Bead  Sphere

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-red  Reddish-purple

Describe the color of this stone  Medium dark, moderately strong, purplish-red

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.

What are the light properties of this stone?  Window: 10%  Light return: 35%  Extinction: 55%

Let’s completely grade a stone  Stone ID  Rhodolite garnet  Stone shape  Pear, broad wings  Stone style  Faceted  Color  Medium, moderately strong, purplish red

Let’s completely grade a stone  Light properties  Windowing  10%

Let’s completely grade a stone  Light properties  Windowing  10%  Light return  45%  Extinction  45%

Let’s completely grade a stone  Overall stone quality:  Good

How about this one?

Now let’s do it with real stones...