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Pavilion Facet Angles and Brilliance

(Text and Photos by Barry Bridgestock)

 

In order for a faceted stone to reach its full potential for brilliance and color, the pavilion, or bottom, facets must be cut at angles which will reflect a maximum amount of light, as if they are little mirrors. This is actually a fairly simple concept, but one that is ignored by many cutting firms which, in my opinion, continue to ruin rare gemrough by cutting shallow pavilions which allow light to pass through the stone instead of being reflected up through the crown.

First, I would like to correct a couple of misconceptions that have come to light(no pun intended) in some of the questions I've received. Light entering through the bottom of a stone does little or nothing to enhance its brilliance or color. In fact, a correctly cut stone will have very little light entering through the pavilion. Also, when a stone is mounted and being worn in a ring or pendant there is really no place for the light to come from. I had to learn this the hard way, naturally. Before I started faceting, I bought my first colored stone, a blue sapphire. It had a nice color when it was in its gembox, resting on a cotton pad. I bought the stone and took it to a goldsmith without ever lifting it up off the cotton pad to examine it. A month later when I picked up my ring, I was shocked by the stone's appearance. The nice blue color was only visible at the very ends of the stone and the rest of it was the Black Hole of Calcutta. When I put the ring on, it only got worse. Lesson learned! You simply cannot improve the appearance of a dark stone by cutting pavilion angles that are shallower than the recommended angles, and since the color you perceive when you look at a stone is being carried by REFLECTED light, shallow angles cannot bring out the color out of a stone.

To demonstrate this principle, I decided to photgraph a shallow, native cut citrine on a cotton pad and then photograph it again off the pad. As you can see, a large portion of the stone has no color at all.

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I then recut the the stone at the correct pavilion angles and took a couple of pictures showing how I "closed the window" that was allowing the light to leak out of the bottom.

Because the new pavilion facets extended quite far into the crown, or top half of the stone, I had to cut a new girdle and then recut the crown. In this process, I lost 44% of the original carat weight even though I had the stone centered perfectly for the recut.

 

The results of this recut speak for themselves, but unfortunately some of the better color zones producing the orange tones were lost in the recutting. This, along with the carat weight loss, is why I don't buy stones for recut! I could have demonstrated the importance of using correct pavilion angles using any type of gemstone. If a gemstone with a higher refractory index and more brilliance than citrine is cut at the wrong pavilion angles, the R.I. becomes irrelevant because it only comes into play if the light is being reflected from the pavilion facets back through the crown facets.

To demonstrate to my middle school students how the transparent pavilion facets can function as mirrors, I used two panes of glass and the Arizona sun. If you hold the panes like a partially opened book with the sun over your shoulder and slowly move the panes back and forth, you will get a reflection of the sun coming from both panes. One of the images is actually being reflected from the other pane. I also used the panes to demonstrate the need for a high polish on the pavilion facets. When I started faceting, I was careless about polishing these facets. After all, who was going to see them? Well, poorly polished pavilion facets do NOT make very good mirrors! This can be demonstrated by using dirty glass panes, observing the reflections, and then repeating the observation using clean glass. This is a fun activity, but if you live in a place like Arizona you might want to wear sunglasses, as the old guy in the photo learned. If you do the window pane demonstration, one thing you will notice is that after you get the sun's reflection from both panes, the slightest movement of the panes in relation to each other will cause the bright reflection to be lost. Similarly, there really isn't much leeway for maximum reflection from pavilion facets. To compound the problem, different types of gems have different maximum facet angles.

Hopefully, this brief discussion on the importance of pavilion facet angles has been coherent and has explained a very important aspect of what goes into the fashioning of a bright gemstone. Crown facet angles will be covered in a separate Gem Topic of the Month column.

 

 


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