Measuring Crown Angles (5.11)
The older original method used by diamond cutters was a star gauge to measure the crown angle from the table, which should -- if the stone is accurately cut -- coincide with the crown’s angle to the girdle plane. In cutting, consistent angles and depths, are a must, to end up with acceptable symmetry. This also leaves a table that is dead center and parallel to the girdle plane. Other methods of determining crown angles will be discussed shortly.
The Scientific Calculator Method
Calculate the table percentage by dividing the table diameter, (average of 4 measurements), by the average girdle diameter.
Subtract the table percentage from 1.
Multiply the result by 50.
Divide the crown depth by the calculated value.
Use the tan-1* function on a calculator to get the crown angle.
*tangent
Proportion Analyzers
The very popular Sarin DiaScan Diamond Cut Evaluator (currently selling for $7400-$9400), is a popular choice for diamond cutters and volume dealers. The Sarine Diamension HD is more sophisticated and detailed instrument. There are also other similar instruments from Sarine.
Left Courtesy of Kassoy: https://kassoy.com/sarin/sarin-diascan-diamond-cut-evaluator.asp
Right Courtesy of Aerodiam: https://aerodiam.com/sarin-products/diamension-hd/
GIA's Facetware: Free access to GIA's database of 38.5 million proportion sets. https://www.gia.edu/facetware
The proportions will be further discussed in subsection 5.42 "Cut Grade"