Light goes into the gem and makes at least one bounce off a pavilion facet, but is not returned back to the observer through the crown. We cover windowing in much greater detail later in this article. This creates gray and dull areas in the middle of the gem (see Fig. The effect is that you are seeing though the gem.
Light passes through the gem and is not bounced back in any direction by the pavilion facets to the observer. Parts of this figure were borrowed from Richard Hughes, Gemological Digest 1988, Vol. This chart illustrates the effect of crown and pavilion angles on a gem’s ability to reflect light.
Virtually the entire spectrum of the visible light passing through the gem is absorbed. 3-01) and light just can’t get through it. Extinction is only one of four different causes of the dark or black portions of the pattern observed in a gem. Darkness-The Good, the Bad, and Its CausesĮxtinction is considered by many in the jewelry trade to be the dark areas seen when looking at the gem face-up. There are many tradeoffs to be considered. Perhaps if cutting that rough into a single gem will result in the largest-cut gem of that species, the cutter may opt for cutting the single gem. However, in some cases, one or the other may be significantly more valuable. Do they cut several small gems from the rough or do they cut a single, larger gem that will be quite shallow and badly windowed? In many cases the resulting value in either scenario above will be about the same. For example, a cutter has a very thin piece of rough that happens to be very rare. A Cutter’s Tradeoffsīefore discussing how the trade assesses relative value of colored gems, it is important to realize that you will often see examples where the cutter had to choose between various tradeoffs. DiamCalc does not show double refraction. 3-09) were made using the program DiamCalc adjustments were made to the refractive index to represent the gem material being demonstrated. Face-up patterns (such as the right side of Fig. 3-09) are from scans of real gems so as to illustrate aspects of gem cutting.
#Depth of extinction impressions series
This web version of the original series is divided into five separate articles and reflects minor stylistic edits to the original.Ībout the images: Wireframes or depictions of facet arrangements (such as the left side of Fig. GIA researcher and cut expert, Al Gilbertson, examines the elements of cutting, investigates the choices and tradeoffs a cutter makes and why, and provides guidelines for assessing various aspects of cut quality for colored gemstones. Originally published in GemGuide in 2016, this comprehensive series examines the quality factors that influence the value of colored gemstones, with a specific emphasis on the role cut quality plays in determining the value of faceted gems. Photo: Orasa Weldon/GIA Part 3 of 5 in the series: Value Factors, Design, and Cut Quality of Colored Gemstones (Non-Diamond) But this kunzite’s deeper proportions results in a small window and a face-up color (top view) that is more saturated than when viewed on its side. Many jewelers might shy away from gems with deep proportions. Comprehensive CAD/CAM For Jewelry Certificate
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