1. Grading Colored Gemstones Carl Wozniak

2. 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

3. 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

4. Which would you buy? $2,100.00 $45.00

5. Stone shapes

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

7. 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

8. 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

9. Color-saturation  Saturation is the strength or purity of a hue  Grayish (brownish)  Slightly grayish  Very slightly grayish  Moderately strong  Strong  Vivid

10. 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

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

12. 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

13. 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

14. 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

15. 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

16. Light return and extinction The dark areas represent light that is lost from the crown of the stone. You estimate its percentage also.

17. 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%

18. 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.

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

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

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

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

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

24. How about this one?

25. Now let’s do it with real stones...