Every stone is scored from the proportions on its IGI certificate before it is listed. Only stones that pass are offered. This page says what the score is made of and what it is not.
What goes in
Four numbers printed on every IGI round brilliant report: table percentage, total depth percentage, crown angle and pavilion angle. Star and lower-half lengths are used when the report gives them. Carat, colour, clarity and the laboratory's cut grade are copied across for display and play no part in the score.
Step one: the gates
A stone must clear every one of these, or it fails whatever else it does:
- Table between 52.5% and 58%.
- Total depth between 60.0% and 62.5%.
- Crown angle and pavilion angle each inside a set range. Crown angles below 33° or well above 35.5° fail; pavilion angles above about 41.5° fail.
- Lower-half facets not over-long.
Laboratory cut grades are looser than this. A stone can carry Ideal, Excellent and Excellent from the laboratory and still fail here; that is the point of grading again.
Step two: do the angles agree with each other?
For any crown angle and table there is a pavilion angle that completes it: light entering through the crown is returned up through the crown rather than leaking out of the pavilion. Tolkowsky's 1919 proportions are one such pair. We compute that complementary pavilion angle for the stone's own crown and table, and measure how far the stone's actual pavilion angle is from it. We call that distance the ridge error. A stone within about a fifth of a degree sits on the ridge; past about a third of a degree the arrows begin to break and the light-return map shows leakage. The ridge is where the score comes from.
Step three: fire or brightness
Along the ridge there is still a choice. Steeper crowns with shallower pavilions throw more coloured flashes; flatter crowns with slightly steeper pavilions return more white light. We give each stone a lean from 0 (brightness) to 100 (fire) and name the result: Fire, Balanced or Brightness. All three are a pass. Neither end is better; they look different under different light, and the Spot light and Studio views are there so you can see which you prefer.
The score
The score runs to 100. It falls with the ridge error and with distance from the proportions we think best. Pass means the stone cleared the gates and was placed as Fire, Balanced or Brightness. Fail means it did not clear a gate. A score can be high and still fail: a stone with a crown angle of 32.9° can sit on the ridge and score in the nineties, and it still fails the crown gate, because at that angle the crown returns too little light in ordinary room light.
What the score cannot see
A certificate averages each angle across the stone. Two stones with identical numbers can differ in how evenly they are cut, in painting and digging of the girdle facets, and in optical symmetry. Those things decide contrast and scintillation, and the score does not know about them. So the score removes stones that cannot perform; it does not rank the ones that can. The full statement is on Questions and method.
Then the pictures
Each listed stone is ray-traced from the same four numbers: studio light, spot light, the Light return map, the Hearts and arrows view, and a short precession loop. They are simulations, labelled as such, and they are what the score is based on, shown rather than described.