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 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

Which would you buy?

$2,100.00$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-redReddish-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

Windowing10%

Let’s completely grade a stone Light properties

Windowing10%

Light return45%

Extinction45%

Let’s completely grade a stone Overall stone

quality:

Good

How about this one?

Now let’s do it with real stones...