Cuttin' Up with Drew - Critical Angle

Hello readers,

People around the world gaze, wonder and in some cases, obsess over how gemstones are cut out of natural stone. Our lapidarists at Emerald Hollow fall into the third category. We each have our own style and a wide range of common and special techniques to creating a magnificent gem that we earned in a trial-by-fire learning environment. The Official Newsletter is a great way for us to reach the audience that has the same passion we do. My main objective has always been education, so enjoy the first installment of Cutting Up with Drew.

Gems do not become great by accident. Careful thought is given to each gemstone before ever being cut. As a faceter, I have a different objective than that of a cabochon artist, or gem carver. I approach clear gems with one thought in mind, in fact a number, known as the critical angle. This crucial angle is the minimum angle at which light reflects internally within a gem and back out of the top, instead of passing straight through. There are endless factors involved with cutting colored gemstones- shape, color(s), size (dimensions), carat weight, inclusions- however none are as important as the critical angle. The critical angle is the reason why a gem has brilliance- its sparkle, its fire, its life!

Unfortunately, the gemstones most people see are cut beneath the critical angle. What do I mean by “beneath” the critical angle? First let me list some of the more well known gems’ critical angles. It is important to note that varieties of the same mineral will have the same angle. Therefore all varieties of quartz have a critical angle of 40°.

If one cuts beneath this critical angle light passes through the stone entirely and a “fish-eye”, or “window”, effect happens.

I know you have seen it before. You may have even admired how the color was rich near the edge of the gem then dissipated to the center. Most of the time this is done on purpose as a way to “belly cut” gems to make them heavier in bulk parcels which drives up cost. Also because the depth of the rough gem is disregarded, otherwise unrealistic dimensions can be achieved. Their color looks

good if you see the group together but if you hold one individually in your hand they will lose their appeal.

Only a few scenarios permit cutting beneath the critical angle: (1) Cutting certain bi-colored stones like ametrine where the color contrast is more important the flash. (2) Cutting gems like rutilated quartz where inclusions are the focal point, which might get washed out in the brilliance. (3) Certain cuts that were designed with a window as a way to create a shape or pattern like a spider or a cat’s face using negative space.

At the Emerald Hollow Mine lapidary the beauty of your gem and how it reflects your inner brilliance is absolutely CRITICAL to us! The luster of your faceted gems will awe and inspire anyone in the room. We want you to walk out and show the world what a gemstone can and should look like!


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