Figure 1 shows the typical case where the break facets are equally-spaced around the circle (360°/16 = 22.5° apart). Their slopes are shown for the main slopes and proportions cited. They are 0.86% and 0.83% deep, respectively, for the pavilion and crown = total of 1.69%.



With a girdle thickness of 1.0% at the thinnest points (midpoints) this gives a thickness of 2.7% at the thickest points (endpoints). Some gemologists report one, some report the other; if they report the thinner one you must add 1.7% to the depth of the pavilion, crown, and girdle to get the correct total depth of the stone.

Index

To cut the facets at accurately-spaced angles around the stone (azimuths), most faceting machines use an ‘index wheel’ (called a ‘click wheel’ by most diamond cutters). This is a notched wheel, like a gear, into which a ‘latch’ falls to lock the rotary position of each facet.

Many colored-stone cutters use a variety of index wheels with different numbers of equally-spaced notches. Figure 2 shows the spacings of common wheels = 32, 48, 64, 72, 80, 96, or 120 notches. With a 32-tooth wheel the mains are cut at 4,8,12,16,20,24,28,32 and the stars at 2,6,10,14,18,22,26,30. Usually the break facets are cut halfway between the mains and stars – at 1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31.



This photo shows an 80-notch index wheel on a Raytech-Shaw faceting machine. This number is used for special cuts



This photo shows a click wheel from a diamond-cutting rig. It has only those notches needed by the brillianteerer for break and star facets.



Photo courtesy of Garry Holloway, 2005 May