Colored Gemstones: Extinction vs Contrast?

[Dragon28]

I read the thread "New to coloured gemstone buying? Read this first!" and it was the first time I heard about Extinction.

On a website that has a section about the effect of extinction, it says "Some gem cutters expressly cut stones so that facets take turns turning off and on, this effect would be considered being cut for contrast, rather than plain extinction."
https://thegemstoneproject.com/tag/extinction/

When a darker area on a gemstone becomes colorful at a certain angle, would that mean it is not really extinction and instead be a contrast effect from the cut?

 

[chatoyancy]

I don’t think extinction is an easy concept to understand. I think extinction means that part of the gemstone is never bright, no matter how you turn it. I thought my pear shaped spessartite had extinction. However, saturated pears look like that until you turn the gem.

 

[MissyBeaucoup]

If the angles are cut properly, light should bounce around inside the gemstone, off the back, and back to your eye. You can lose these light beams if there is a window, where the light goes out through the back. You may have windows in shallow gems. Or you may have extinction, which is as if the light goes in a bucket and can’t get out. You might see this if a gem was cut to preserve as much weight in the belly as possible (which might give a darker color too). That is why good cutting is so essential.

Then there are also tilt windows and gemstones that show different colors from different directions, one of which may be black or very dark. That could be cool!

 

[suzanne2]

When a darker area on a gemstone becomes colorful at a certain angle, would that mean it is not really extinction and instead be a contrast effect from the cut?

My guess is that *contrast* is deliberately cutting so extinction (turning black) happens facet by facet, alternating. I don't think a darker area can turn colorful. If it could, the cutter would never opt for extinction. Actually, I'm just grabbing this out of thin air, hoping that a cutter might jump in. Gene@PrecisionGem are you around? This is interesting.

 

[PrecisionGem]

My guess is that *contrast* is deliberately cutting so extinction (turning black) happens facet by facet, alternating. I don't think a darker area can turn colorful. If it could, the cutter would never opt for extinction. Actually, I'm just grabbing this out of thin air, hoping that a cutter might jump in. Gene@PrecisionGem are you around? This is interesting.

Extinction is not a black or dark spot in the gem material, it's an area of the stone that is not reflecting light given the position of your eye, and the location or source of the lighting. Generally, the more even the lighting the less extinction you will see. Gems are often best viewed outdoors in light shaded from direct sunlight.

As a cutter, we strive for designs that limit extinction when the stone is viewed perpendicular to the table. Once you start to rock the stone, then different facets may start to reflect light, and the area's of extinction move, but evaluation of a cut is done looking perpendicular to the table.

Often on here, I will see someone commenting on a stone, thinking there are black spots in the stone, when the reality is, due to the photo, it's just facets that are at that angle not reflecting light.
What makes it hard from a photo to judge is you don't really know the light source, and often people don't photograph the stones with the camera lens perpendicular to the table of the stone.

 

[PrecisionGem]

Here is a ray trace of a design I often cut. The black areas are extinction, notice as the stone rocks, the areas of extinction change. (If the gif file works!)

 

[mayday]

This gif is mesmerizing! Thanks for the lesson!