Gemstone Grading Brilliance/Internal Luster

Brilliance is sometimes called “Internal Luster”. The brilliance of precious gems is an often neglected factor in colored gem grading. And that’s strange, because the brilliance in a gemstone is probably the single most important element.

Besides being a significant contributor to a gem’s beauty and appeal, observing flashes of brilliance is extremely important as a signal of other characteristics.

For in those flashes of brilliance we see the true huetone and saturation .. and even get a strong clue about clarity.

Observing Brilliance in Colored Gems

  • Hold the gem with tweezers and about 45 degrees to the light path
  • If the gem is not round, only half will normally show brilliancy in the illustrated orientation
  • First observe the lower half [Item 1] on [a] half of the gem
  • Rock the gem a few degrees to get the best play of brilliance
  • Note: When moved, the brilliancy flashes will seem to trade places with similar areas of extinction. That is perfectly normal.
  • Mentally estimate the total area of brilliance and extinction as a percentage for that half of the gem
  • Caution: Don’t rock the gem more than about 15% or the entire gem will appear washed out
  • Exclude any areas of the gem (table and crown) that never show brilliance
  • Rotate the gem 180 degrees and repeat the procedure [Item 2] on [b] that is now the lower portion of the gem
  • Rock the gem slightly to find the maximum % of brilliance
  • Estimate the percentage for that half of the gem
  • Average the two percentages to get a total percentage of brilliancy

Round gems are easier to grade for brilliance. One observation is normally sufficient to establish a percentage of brilliance.

If 100% of the gem shows brilliancy in both positions when rocked it is considered 100% brilliant

Note: A gem can never show 100% brilliancy in all areas at the same time

Germstone Brilliance Signals

  • Fuzziness – an indication that a precious gem is heavily included which tends to diffuse the transmission of light
  • Dead areas are due to the masking of light transmission and indicate inclusions
  • Dimness or limited transparency, again, indicates inclusions
  • Weakness of points of brilliance in a clean stone usually indicates a poorly proportioned gem
  • Imbalance in the areas of brilliance in a clean stone normally indicates poor symmetry

Grading Points for Gem Brilliance

Make an estimate of the total area that shows brilliance and assign a number grade based on that percentage. Example. If half the stone is brilliant, that equated to 50%, so assign a preliminary grade of 5.

  • 10% = 1, 20% = 2, 30% = 3, 40% = 4, 50% = 5, 60% = 6, 70% = 7, 80% = 8, 90% = 9, 100% = 10
  • Deduct 1 if the brilliancy areas are not crisp
  • Deduct 1 if the brilliancy areas are unbalanced
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