The same grinding wheel that cuts a facet into a diamond also polishes it, the difference is in the diamond powder grain size used and the speed and pressure of the operation. Cutting uses coarser diamond powder at higher pressure and lower speed to remove material quickly. Polishing uses finer diamond powder at lighter pressure and higher speed to refine the surface to a mirror finish. The skill of the polisher lies in reading the grain of the diamond crystal, the directions in which it polishes easily versus those in which the surface resists and may become rough or "burned". Polishing against the grain does not produce a mirror surface; it produces a characteristic surface defect that a gemologist can see at 10× magnification and that GIA records on the certificate. : GIA Diamond Grading documentation, gia.edu/diamond-grading; GIA Gem Reference Guide (2006 edition), pp. 42–45

The polishing wheel: schyf and diamond powder

Diamond polishing is performed on a horizontal spinning wheel, historically a flat disc called a schyf (from the Flemish word for wheel), made of cast iron or other metals. The schyf is charged with diamond powder (typically applied in an oil or water suspension) that embeds in the metal surface and abrades the diamond facet as it is pressed against the spinning wheel. The particle size of the diamond powder used for polishing (typically a few microns) is much finer than that used for faceting (tens of microns), producing progressively smoother surface finishes (GIA Gem Reference Guide, 2006, pp. 42–45; historical and modern polishing methodology).

Grain direction: the critical variable

Diamond has a cubic crystal structure with defined hard and soft grain directions, orientations in which abrasion proceeds easily (soft directions) versus slowly or not at all (hard directions). Polishing must be done in a soft direction for each facet; polishing against the grain is extremely slow and produces surface defects. The polisher must orient each facet to present a soft direction to the wheel, a judgment requiring knowledge of the stone's crystal orientation, which is established during the planning and blocking stages (GIA Gem Reference Guide, 2006; diamond crystallography documentation).

Polish grade on the GIA certificate

GIA grades the polish of every facet on a round brilliant Diamond Grading Report, using a scale of Excellent, Very Good, Good, Fair, and Poor. Polish is assessed under 10× magnification. Specific polish defects that result in grades below Excellent include: polishing lines (fine parallel lines visible under magnification caused by abrasion in a slightly off-grain direction); lizard skin (a wavy surface texture from polishing in an incorrect direction); burn marks (heat-damaged surface areas from friction); and abrasion (rough, scratched facet edges). A GIA Excellent polish grade indicates facets that are completely free of these defects under 10× magnification (GIA Diamond Grading documentation, gia.edu/diamond-grading).

Primary sources

GIA Diamond Grading documentation. gia.edu/diamond-grading. Gemological Institute of America. [Cut grade system; proportion parameters; polishing and symmetry assessment; planning methodology context.]

GJEPC (Gem and Jewellery Export Promotion Council). gjepc.org, Mumbai. [India cutting industry data; Surat manufacturing statistics; export figures.]

Reinitz, I. et al. (2006). "Development of the GIA Diamond Cut Grading System." Gems & Gemology, 42(3), GIA. [Cut grade system basis; proportion parameters and their effect on light performance.]

Sarine Technologies product documentation. sarine.com. [Galaxy family scanning instruments; planning software; mapping of inclusions and proportions for yield optimisation.]