The optimal viewing distance from the light source should be approximately 18” to 20” (50 cm).

The gem being examined may be loose or mounted. If loose, it should be held in tweezers or a tray so that it can be held close to individual color windows.

The observer should pass the stone from left to right or vice versa, to locate the column of color that best matches the gem. Keep in mind that the best match may fall between two columns of color.

Then the observer should pass the stone vertically to establish which color window best represents the intensity of color exhibited by the gem. It can also be beneficial to turn the sheets around so that the color progresses in the opposite direction when needed. Again, keep in mind that the best match may fall between two color windows.

To test for an element of tone (i.e. gray or brown) the observer should also try different overlays until the best match for the color of the gem is located.

The numeric value should then be noted: e.g. 10 – 13