How To Grade Gemstones

It is a common perception that people think that only diamonds have grading. However, they do not know that there is also a set of criteria to figure out the grade of the gemstones.

Following are the basics to determine the gem’s grading:

1. Color of gems

Basically, the value of the gem is based upon its color. Natural gemstone usually reflect tertiary colors(red-orange, yellow-green, or blue-violet) and zero percent saturation( Mainly, a saturation is a state where color contains 0% of gray).

2. Color Grade

A color grade  illustrates  the  composition of  main color  as compared  to other visible colors in the stone. A stone with 100% color grade consider falls in the category of  fine quality. Following is a  measurement scale:

     Excellent: 1-2

      Very Good: 3-4

      Good: 5-6

      Fair: 7-8

      Poor: 9-10

 3. Color tone

 Any color grade is seen in combination with the tone. In a rare case,  a colored stone could be white (light 5) or black (Dec 95) with just a hint of color. Basically, it reflects the lightness or darkness  of the stone.  If a stone is too light in tone, the color will not be rich enough. In a vice versa case, if a stone is too dark, then it will  be devoided with transparency and brilliancy.

 Tone Scale :

 Very Light: 0-20

 Light: 25-35

 Light-Medium: 40-50

 Medium: 55-65

 Medium-Dark: 70-85

 Very Dark: 90-100

4. Color Zoning

 There  are various  stones, which  depicts colors only in partial area or layers.  Following  is a measurement  scale  to determine  the traits  as per  the color zoning.

 None : The color is  distributed  in the equal proportion.

 Faint :  it  depicts the visible changes in color saturation.

 Gradual : Gradually  weaken of color in certain part.

 Visible :  Stone has clear color patches or layers.

5. Clarity

 It  is yet another factor that determines the value of the stone. Generally, clarity is judged with a 10x lens. Commonly, it is termed  as “Free of Inclusions” over “Lightly”, “Moderately”, “Heavily” to “Excessively Included “.  Following is a measurement scale for the clarity:

Free of Inclusions: FI

Lightly Included: LI1-LI2

Moderately Included: MI1-MI2

Highly Included: HI1-HI2

Excessively Included: E1-E2-E3

 6. Depth

 It includes the height of the stone divided by its minimum width. Ideally, it lies between the range of  60% and 80%. Mainly, the depth is decided by the given shape of the rough stone. Stones below the 50% is called shallow. It is hard to find a maintain saturation in a light tone shallow stone.

7. Cutting Grade

 It  is a numeric number that describes the proportions and geometry of a gem.  The finish grade reflects the polish of a gem.  Following is a scale:

Excellent: 1-2

Very Good: 3-4

Good: 5-6

Fair: 7-8

Poor: 9-10

Accepted  parameters

  Carat Weight:  1 carat or larger

  Color: Good (6) or better

   Tone: It is the most accepted  tone for ruby, sapphire and emerald ranges from medium to        dark (65-85).

   Clarity: Moderately Included Two (MI2) or better

   Average Brilliancy: 50% or more

  Cutting/Finish: Good (6) or better

Following is list of laboratories that offer Gemstones Grading :

  • International Gemological Institute (IGI), independent laboratory for grading and evaluation of diamonds, jewellery and colored stones.
  • Gemological Institute of America (GIA), the main provider of education services and diamond grading reports.
  • Hoge Raad voor Diamant (HRD Antwerp), The Diamond High Council, Belgium is one of Europe’s oldest laboratories.
  • American Gemological Society (AGS) is not as widely recognized nor as old as the GIA.
  • American Gem Trade Laboratory which is part of the American Gem Trade Association(AGTA), a trade organization of jewelers and dealers of colored stones.
  • American Gemological Laboratories (AGL)
  • European Gemological Laboratory (EGL)
  • Gemmological Association of All Japan (GAAJ-ZENHOKYO), Zenhokyo, Japan, active in gemological research.
  • Gemmological Institute of Thailand (GIT) is closely related to Chulalongkorn University
  • Gemmology Institute of Southern Africa, Africa’s premium gem laboratory.
  • Asian Institute of Gemmological Sciences (AIGS), the oldest gemological institute in South East Asia, involved in gemological education and gem testing.
  • Swiss Gemmological Institute (SSEF), focusing on colored gemstones and the identification of natural pearls.
  • Gubelin Gem Lab, the traditional Swiss lab.

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