The Pavilion Main Method (5.14)
The Pavilion Main Method is another effective way to estimate crown angles without the use of instruments. The elongated kite-shaped “pavilion main” can be viewed through the crown with the table facing toward you. You will see shape distortion where the image of the pavilion main passes under the kite-shaped bezel (top) main facet. The amount of distortion indicates the approximate angle of the crown main facets. Where the pavilion main passes under the crown main, you will notice that the image expands to a wider shape. At 34.5°, it is double the width from the table side to the crown main facet side. As the crown angles decrease, the thickened pavilion main shrinks. At less than 25°, there is virtually no change in shape. This image distortion is caused by the crown angle’s refractive effect. It bends the image projection toward the center or culet of the diamond. With extremely high crown angles, 39°+, the image is a kite shape itself.
Typically you will also note misaligned crown and pavilion facets when using this method of crown angle determination. This makes it a little more challenging to use, and it also points out a non-symmetrical stone.
Eventually you will have the ability to estimate crown angles by looking closely at the side profile of the diamond. Note: When viewing a side profile, it’s important to remember to be very careful when holding a diamond in tweezers when the culet is contacting the tweezer’s metal surface. A culet or extremely thin girdle are easily broken even with light pressure.
Many labs use pavilion depth percentage instead of pavilion angle. The correlation between them is important to understand. In addition to correct pavilion angles, consistent pavilion angles are also important to achieve proper symmetry in the finished diamond. You will often see diamonds with off-center culets. This is caused by the cutter’s choice in leaving the pavilion angles inconsistent. For example, one side is 39.5˚ and its opposite is 42.5˚ allowing the culet to favor the higher angled side.
An off-center culet may also be caused by inconsistent depths of the pavilion main facets. This is obvious when viewing the girdle plane, which should run parallel to the table.