Proportion Analysis
Proportion Analysis
Systematic measurement of a faceted stone's dimensions and angles
Proportion analysis is the systematic measurement and recording of a faceted gemstone's dimensional ratios and facet angles, conducted as the technical foundation for formal cut grading. The standard parameter set includes table percentage, crown angle and crown height percentage, pavilion angle and pavilion depth percentage, total depth percentage, girdle thickness, culet size, and, for fancy shapes, length-to-width ratio and shape-specific descriptors.
Instrumentation
Modern laboratories use non-contact optical scanners — Sarine, OGI, Helium, and equivalent platforms — that capture three-dimensional models of the stone and compute every standard proportion to within hundredths of a degree. Older bench methods using callipers, profile projectors, and reflected-image gauges remain in use for spot checks and for fancy-shape assessment. GIA reports proportion data graphically on every diamond grading document, and AGL Prestige reports include analogous data for coloured stones above the threshold size.
Use in cut grading
Proportion analysis is the input to every cut grading algorithm, but it is not the whole picture. GIA's diamond cut grade combines proportion data with symmetry assessment, polish assessment, and proprietary scoring of brightness, fire, and scintillation against ranges established through perception studies. AGS Laboratories runs a parallel light-performance computation directly on the scanned model. For coloured stones, proportion data is recorded but the lack of an industry-standard cut grade means trade interpretation varies.
In practice
Dealers and buyers reading a grading report should treat proportion data as a diagnostic tool: the numbers identify whether a stone falls within ranges that produce strong optical performance for its species and shape. For diamond, the round-brilliant ranges are tightly defined and well documented; for coloured stones and for fancy diamond shapes, the ranges are wider and the trade eye remains essential.