With grateful acknowledgment to John Pollard, Sergey Sivovolenko, AGSL and GIA Laboratories for valuable resources and input. Graphics produced with DiamCalc software by OctoNus.
Note: This article pertains only to 'superideal' diamonds, meaning those with commonly agreed 'ideal' proportions and a high level of optical symmetry. Our understanding is that this diamond category makes up less than 1% of all round brilliant diamonds produced. The effects can be quite different on deeper and shallower diamonds.
Preface: Common Questions
Q: What are painting & digging?
A: They are approaches taken in final stages of polish, or brillianteering.
Painting shifts upper/lower girdle facets toward the mains so angles are closer together. The result is that average girdle thickness where the half junctions meet is greater than average thickness where the mains meet (diagram below).
Digging tilts upper/lower girdle facets away from mains so angles are farther apart. The result is that average girdle thickness where the half junctions meet is thinner than average thickness where the mains meet (diagram below).
For a more technical description refer to this GIA publication: http://www.diamondcut.gia.edu/pdf/6_05_RDR_pg239_243pdf.pdf
Q: Why Paint or Dig?
- To try and retain weight at critical points: If a diamond is close to a commercially important weight the painting approach may be used because less material is polished away. On the example below normal indexing would result in a finished diamond weighing 0.99 ct. Painting to 4 degrees - particularly on the pavilion's long lower girdle facets - allows the commercially important 1.00 ct mark to be 'saved.'
- To improve a verbal description of girdle thickness or clarity: For example, a very thick girdle may be dug out enough so that it will appear only 'slightly thick' to the grader, or an inclusion could be dug-out in order to improve the clarity grade.
- Relative to crown-only painting: To acquire desirable visual properties. A measure of crown-only painting on 'superideal' diamonds can improve the diamond's brightness and increase the amount of visible broadfire dispersion.
Q: Why is it said that Painting & Digging are bad?
A: Because the most common uses of painting & digging are 'swindling strategies' in answers 1 and 2, above. What is not commonly known is that a measure of crown-only painting on superideal makes can be used to acquire desirable visual properties. Remember that the 'superideal' category of diamonds makes up less than 1% of all round diamonds produced.
The purpose of this article is to illustrate the differences between crown-only painting (which can be positive) and pavilion painting or digging of any kind (which are negative).