Every stone we list is "triple Excellent" on its laboratory report: Excellent cut (IGI prints Ideal for its top grade), Excellent polish, Excellent symmetry. We treat that as the floor, not the standard. This page says why, and shows it.
What the three grades measure
- Polish is the surface finish of the facets. Excellent means no polish lines or burn marks visible at 10× magnification.
- Symmetry is how well the facets are placed: a centred table, facets that meet where they should, a girdle of even thickness. It is a workmanship grade. It is not the same thing as a hearts-and-arrows pattern, which depends on the facet angles being consistent around the stone.
- Cut grade is the laboratory's judgement of the proportions as a whole. Each laboratory keeps its own scheme; all of them award the top grade to a wide range of proportion combinations.
Polish and symmetry say the cutter was careful. Only the cut grade says anything about how the stone handles light, and it says it loosely.
Why the top cut grade is not enough
The envelope of proportions the trade cites for a top cut grade runs roughly from 31.5° to 36.5° of crown angle, 40.6° to 41.8° of pavilion angle, and 52% to 62% of table (the laboratories weigh combinations, so not every corner of that envelope is graded Excellent, but much of it is). That is five degrees of crown and more than a degree of pavilion. Across that envelope the way light behaves changes completely.
What the grade does not require is that the crown and pavilion agree with each other. For any crown angle and table there is one pavilion angle that returns light entering the crown back out through the crown instead of leaking it. A steep crown on a steep pavilion, or a shallow crown on a shallow pavilion, can each sit inside the envelope and still miss that line by close to a degree. Our grader measures exactly that distance; How we grade explains it.
We ran our grader over every combination inside that envelope at the laboratories' reporting steps: 0.1° of crown and pavilion, 1% of table, with a normal girdle. About one combination in seven clears our gates. Scores run from 0 to 100, and half the envelope scores below 77.
See the two worst stones that still fit the envelope
These are simulated stones, built from numbers at the edges of the envelope. Both would carry Excellent polish and symmetry; both score 0 on our grader.
- Steep on steep: crown 35.5°, pavilion 41.8°, table 59%, depth 62.8%. The pavilion is more than a degree steeper than the crown calls for; light entering the table leaks out under it. In the hearts-and-arrows view the arrows thicken and blur. Compare it with a stone that scores 100.
- Shallow on shallow: crown 31.5°, pavilion 40.6°, table 62%, depth 58.0%. Big and bright face-up under a shop's spotlights, with almost no fire and a flat, glassy look in ordinary light. Compare it with a bright stone that scores 100.
In the comparison, look at the hearts-and-arrows view first: a stone whose crown and pavilion agree shows eight clean arrows table-up and eight clean hearts table-down; the mismatched stone does not, even though its symmetry grade is Excellent. Then switch to Worn light, which is where most of a ring's life is spent.
What we do instead
We start from triple Excellent because it removes careless cutting. Then we score the proportions on how well the crown and pavilion agree, and we list only stones that pass. A stone that scores 95 or more we mark Exceptional: its angles sit almost exactly on the line. None of this is a laboratory grade, and none of it sees what a certificate does not record; the Questions and method page says what the simulation can and cannot show.