The 96 index — also referred to as the 96-tooth index or 96-index gear — is the most widely adopted indexing standard in precision faceting. Mounted on the mast of a faceting machine, the gear divides a full 360° rotation into 96 evenly spaced positions, each representing an angular increment of 3.75°. Because the overwhelming majority of published facet diagrams are written to 96-index notation, the gear functions in practice as a universal reference system: a cutter anywhere in the world reading index position 48 on a diagram knows precisely where to place the next facet.

Mechanical Principle

An index gear is a toothed disc — typically machined from brass or aluminium for dimensional stability and resistance to wear — that locks the dop (the stone-holding spindle) at discrete, repeatable angular positions. A spring-loaded detent pin seats into each tooth notch, holding the spindle firmly while the lap rotates beneath the stone. The 96-tooth configuration is chosen because 96 is divisible by a large number of integers: 2, 3, 4, 6, 8, 12, 16, 24, 32, and 48, among others. This mathematical richness means that virtually every symmetrical cut — round brilliants, square cushions, emerald cuts, ovals, trillions, hexagonals, and eight-fold fantasy designs — can be expressed cleanly in whole-number index positions without remainder or approximation.

Comparison with Other Index Standards

Two other indexing standards remain in circulation, principally on older or specialised machines:

• 64-index: Each position spans 5.625°. Common on some vintage American and Australian machines, the 64-index accommodates four-, eight-, and sixteen-fold symmetry but cannot reproduce the three- or six-fold divisions that a 96-index handles cleanly. Cutters working from 96-index diagrams on a 64-index machine must use published conversion charts or accept slight angular compromises.
• 80-index: Each position spans 4.5°. Less common than either the 64 or 96, the 80-index accommodates five- and ten-fold symmetry (pentagons, ten-mains) more naturally than the 96, but is rarely encountered outside specialist cutting circles.

Conversion between standards is straightforward in principle — multiply the 96-index position by the ratio of the target gear's tooth count to 96 — but fractional results require rounding, which introduces small angular errors that accumulate across a multi-facet design. For this reason, most contemporary machine manufacturers and diagram publishers have standardised on 96 as the default, and adapters or software-based conversion tables are treated as workarounds rather than primary tools.

Reading a 96-Index Diagram

Published facet diagrams present index positions as integers from 1 to 96 (or, in some conventions, 0 to 95). By convention, position 96 (or 0) aligns with a reference point — typically the top of the stone as oriented in the dop — and numbers advance clockwise when viewed from the culet end of the spindle. A standard 57-facet round brilliant, for example, places its eight main pavilion facets at positions 6, 18, 30, 42, 54, 66, 78, and 90 on a 96-index gear, yielding perfect eight-fold symmetry with each main separated by exactly 12 positions (45°). The girdle facets and star facets fill the intervening positions according to the specific diagram.

Materials and Construction

Brass remains the preferred material for precision index gears because its machinability allows tight tooth tolerances and its density damps vibration. Aluminium gears are lighter and adequate for amateur or hobbyist machines where absolute positional precision is less critical. On high-end production machines, the detent mechanism may incorporate a hardened steel pin and a replaceable brass insert at each tooth notch to extend service life. Gear diameter varies by manufacturer but commonly falls in the range of 75 to 100 millimetres; larger diameters improve angular resolution by increasing the arc length between adjacent teeth, reducing the effect of any play in the detent.

In the Trade and Cutting Community

The 96-index standard is effectively assumed in the international amateur and professional faceting community. Major design repositories — including those maintained by the United States Faceters Guild and contributors to the Faceter's Digest — publish diagrams almost exclusively in 96-index notation. Faceting software such as GemCad and Gem Cut Studio generates cutting sequences referenced to 96 by default, though both programmes can output to alternative gear counts. For a cutter acquiring their first machine, selecting a 96-index-compatible instrument is strongly advisable: the breadth of available diagrams, the ease of finding reference material, and the ability to compare notes with other cutters all depend on shared positional notation.