Gemstone Grading

GemstonesThe grading of colored gemstones by the Gemological Institute of America is actually more complicated than that of diamond grading. Here is an overview, which applies primarily to transparent colored stones:

Clarity

First of all, clarity is divided into three types, according to how many inclusions a specific gemstone characteristically exhibits.

Type I – often virtually inclusion-free

  • aquamarine
  • blue topaz
  • green tourmaline
  • blue zircon
  • tanzanite

Type II – usually included

  • alexandrite
  • ruby
  • sapphire
  • garnets
  • amethyst
  • citrine
  • peridot

Type III – almost always included

  • emerald
  • red, pink, and watermelon tourmaline

Within each of these types, stones are then defined by clarity grades VVS, VS, SI1 & SI2, and I1, I2 & I3. The definitions of the grades vary from type to type.

Colors

Color is divided into three categories: Tone, Hue, and Saturation.

  • Tone is the lightness/darkness of color ranging from 0 (lightest) to 10 (darkest). Most stones are between 2 and 8.
  • Hue is the actual spectral color of the stone.
  • Saturation is the strength or purity of the hue, which ranges from “grayish” or “brownish” (1) to “vivid” (6).

Cut

This is much the same as the grading of the cut of a diamond. Taken into consideration are: profile, table, size, girdle thickness, total depth, crown height/pavilion depth, bulge (step cuts) and balance. Finish includes polish, symmetry and facet survey.

Carat Weight

All colored stones are weighed in carats. Remember though, that all minerals have different densities. Therefore, two stones may weigh exactly the same, but not be the same size.

* Important things to remember:

Color is THE most important component of selecting a colored gemstone. Find the color that appeals to YOU the most.

There are no “ideal” proportions for colored gemstones. Weight retention is usually of primary concern to the cutter. “Face up” is the most important to you, the consumer.

Colored gemstones can also display some very unique optical characteristics. Among them: play-of-color (opal), adularescence (moonstone), asterism (star sapphire or ruby) and chatoyancy (cat’s eye chrysoberyl). If you are looking for something different for your jewelry collection, consider one of these phenomenal gems.