Pavilion Angle Gauge — The Lapidary's Angle Reference
Pavilion Angle Gauge — The Lapidary's Angle Reference
Mechanical or digital gauge used to measure or verify the pavilion angle of a faceted stone
A pavilion angle gauge is a mechanical or digital tool used by lapidaries, faceters, and gem assessors to measure or verify the angle between the pavilion main facet and the plane of the girdle on a faceted stone. It is one of the basic cut-quality measurement tools at the bench, alongside the table gauge and crown angle gauge, and is used both during faceting (to verify that each pavilion main is being cut to the intended angle) and after the fact (to assess cut quality on stones submitted for grading or appraisal). Several formats are in common use, ranging from simple mechanical templates to digital systems with sub-degree precision.
Formats
Traditional mechanical gauges are pre-cut metal or plastic templates with a series of angled edges; the lapidary places the gauge against the pavilion main facet of the stone (the stone is held in a dop or fixture, the gauge against the facet at the girdle line) and reads the angle by matching the gauge's pre-marked angle to the facet's slope. Resolution on these templates is typically 0.5° or 1°, sufficient for routine work but coarse for cut-grade assessment.
Contact gauges with continuously adjustable pointers — small protractor-style instruments that rest on the girdle and read the pavilion angle through a calibrated dial — provide finer resolution, typically 0.1° or better. Digital gauges replace the dial with an electronic readout from a contact probe or optical sensor and are used in production faceting and laboratory grading. The most precise format is the non-contact reflectometer (Sarine, OGI, and similar), which measures pavilion angle by analysing reflected light patterns and is the standard for laboratory cut-grading.
Use during faceting
The pavilion angle gauge's first role is in faceting itself. The faceter sets the machine's mast to the target pavilion angle, cuts the first pavilion main, and verifies the cut against the gauge before proceeding to the remaining seven pavilion mains (in a round brilliant). Verification at this stage catches errors before they propagate across all eight facets. After cutting all pavilion mains, the gauge is used again to verify symmetry — that all eight facets are at the same angle within the machine's tolerances — and to identify any facet requiring touch-up.
Use in grading and assessment
For grading and appraisal, the gauge is used to verify that the pavilion angle of a finished stone falls within the cut-grade range for the material and stone shape. Cut-grade assessment combines pavilion angle with crown angle, table percentage, total depth percentage, girdle thickness, and other measurements; the pavilion angle is one of several inputs but, for the reasons covered in the pavilion angle reference, often the most consequential. See also pavilion angle, pavilion, faceting, faceting machine.