The Hanneman System
A Basic Course
The cut grade of any round brilliant diamond can be derived from the knowledge of four measurements, two equations, and a table of 12 numbers. The measurements are total height, diameter, crown angle, and pavilion angle. The equations are C/P=0.84 and H=58.3 +3. The table is printed below.
This information can also be used to evaluate the relative severity of any commercial grading system or laboratory. Simply plug in their numbers and compare the results.
Lesson 4: Evaluating Cut
To begin, we must first define our categories, Good, Better, and Best. We have already disposed of Junk and Garbage. Now, we need to determine what is called the C/P ratio. To do this, simply divide the value of the crown angle of the stone by the value of its pavilion angle. This should give you a result somewhere between 0.79 and 0.91. If it doesn’t, you have made a mistake in arithmetic.
Now, consult the table below to convert that C/P value into the cut category of your stone. That is all there is to it.
C/P Ratio Cut Category
0.83 – 0.85 Best
0.81 – 0.83 or 0.85 – 0.88 Better
0.79 – 0.81 or 0.88 – 0.91 Good
Less than 0.79 or More than 0.91 Junk or Garbage
Many stones which have been previously classified as Junk or Garbage on the basis of Total Height will indeed show C/P ratios between 0.79 and 0.91, if you were to measure them. Don’t, it will only confuse you. They have already been eliminated from consideration. Merely having the “correct” C/P ratio is not enough.
The Problem and Solution
From the preceding it is obvious one can convert any Total Height and C/P ratio into a cut grade. The grades above represent perhaps the most stringent cut grading system yet devised. As previously shown, it is more stringent than the present GIA system or any other system in general use today. Therein lies the problem. A cut rating of “Good” does not have an unambiguous definition clearly understood by everyone, everywhere. In simple terms, it is subjective and that makes it useless.
Time and experience have shown there is no simple way of resolving that problem. The only solution lies in abandoning the use of subjective terms and replacing them with purely objective numbers. This can and should be done for all future cut grading reports. The simple act of reporting the Total Height as % of diameter and the C/P value of the stone in question will accomplish this. With that information, anyone can cut grade any stone without ever seeing it—as has just been shown. Reporting these two values would not require any change in present practices. Graders could still use their subjective terms and customers would still be confused and wondering why one grader calls their stone’s cut “Excellent” while another calls it only “Very Good”. However, now, anyone can simply look at the C/P ratio and Total Height values and instantly recognize both graders evaluated that stone identically. Only the names of the categories they used to subjectively describe the cut of that stone were different.
Advantages of Reporting C/P
Current cut grading systems measure the pavilion and crown angles to the nearest 0.1 degree. Since the C/P value is a ratio, there is no problem calculating that value to three decimal places.
Now suppose a laboratory has arbitrarily chosen to define their “Very Good” category as having the “correct” Total Height and a C/P ratio of 0.800 to 0.820, and they receive two stones which have C/P ratios of 0.801 and 0.819 respectively. Certainly, the lab could simply report both stones as grading “Very Good”. However, that is not the whole story.
The first stone is what one could call “barely Very Good” while the other could be described as “almost Excellent”. All other things being equal (which they never are), there should be some sort of a value differential between the two stones. If so, I believe it would behoove the laboratory to convey that information about the two stones to their customer. Using C/P ratios, that can be easily done. Using the present system, it is impossible. Essentially, reporting C/P ratios can increase the precision of one’s cut grade by an order of magnitude. That is indeed worth doing.
Anyone doubting the superiority of this system is invited to try it on their own previously graded stones. Besides the simplicity and ease of determining the final cut grade, you will find the results (adjusted for the less stringent requirements of your present system) are essentially identical. As said in the beginning, “This information can be used to evaluate the relative severity of any commercial grading system or laboratory. Simply plug in their numbers and compare the results.”