Editing Diamond cut (section)

== Theory ==
[[File:Cleaving a diamond, 1946.jpg|thumb|A diamond cutter apprentice cleaving a diamond prior to cutting it; using a steel wedge-like blade and a small [[Club (weapon)|club]], supervised by a senior cutter in the Netherlands 1946.]]
In its rough state, a diamond is fairly unremarkable in appearance. Most gem diamonds are recovered from secondary or [[alluvial]] deposits, and such diamonds have dull, battered external surfaces often covered by a gummy, opaque skin—a comparison to "lumps of [[washing soda]]" is apt. The act of polishing a diamond and creating flat facets in symmetrical arrangement brings out the diamond's hidden beauty in dramatic fashion.

When designing a diamond cut, two primary factors are considered. Foremost is the [[refractive index]] (RI) of a diamond, which, at 2.417 (as measured by [[sodium light]], 589.3 [[nanometer|nm]]), is fairly high compared with that of most other gems. Diamond's RI is responsible for its brilliance—the amount of [[wiktionary:incident|incident]] light reflected back to the viewer. Also important is a diamond's [[dispersion (optics)|dispersive]] power—the ability of the material to split white light into its component [[spectral color]]s—which is also relatively high, at 0.044 (as measured from the B-G interval). The flashes of spectral colors—known as fire—are a function of this dispersion, but are, like brilliance, only apparent after cutting.

Brilliance can be divided into the definitions external brilliance and internal brilliance. The former is the light [[Reflection (physics)|reflected]] from the surface of the stone—its [[Lustre (mineralogy)|luster]]. Diamond's ''adamantine'' ("diamond-like") luster is second only to [[metal]]lic (i.e., that of [[metal]]s); while it is directly related to RI, the quality of a finished gem's polish determines how well a diamond's luster is borne out.

Internal brilliance—the percentage of incident light reflected back to the viewer from the rear (pavilion) facets—relies on careful consideration of a cut's interfacial angles as they relate to diamond's RI. The goal is to attain [[total internal reflection]] (TIR) by choosing the crown angle and pavilion angle (the angle formed by the pavilion facets and girdle plane) such that the reflected light's [[angle of incidence (optics)|angle of incidence]] (when reaching the pavilion facets) falls outside diamond's critical angle, or minimum angle for TIR, of 24.4°. Two observations can be made: if the pavilion is too shallow, light meets the pavilion facets within the critical angle, and is refracted (i.e., lost) through the pavilion bottom into the air. If the pavilion is too deep, light is initially reflected outside the critical angle on one side of the pavilion, but meets the opposite side within the critical angle and is then refracted out the side of the stone.<ref>{{cite web|title=Crown and Pavilion Angles|url=http://www.pricescope.com/wiki/diamonds/diamond-crown-and-pavilion|website=www.pricescope.com|publisher=PriceScope|access-date=28 November 2016|date=2009-04-08}}</ref>

The term scintillation brilliance is applied to the number and arrangement of light reflections from the internal facets; that is, the degree of "sparkle" seen when the stone or observer moves. Scintillation is dependent on the size, number, and symmetry of facets, as well as on quality of polish. Tiny stones appear milky if their scintillation is too great (due to the limitations of the [[human eye]]), whereas larger stones appear lifeless if their facets are too large or too few.

A diamond's fire is determined by the cut's crown height and crown angle (the crown being the top half of the stone, above the girdle), and the size and number of facets that compose it. The crown acts as a [[prism (optics)|prism]]: light exiting the stone (after reflection from the pavilion facets) should meet the crown facets at as great an angle of incidence from the [[Normal (geometry)|normal]] as possible (without exceeding the critical angle) in order to achieve the greatest fanning out or spread of spectral colors. The crown height is related to the crown angle, the crown facet size, and the table size (the largest central facet of the crown): a happy medium is sought in a table that is not too small (resulting in larger crown facets and greater fire at the expense of brilliance) or too large (resulting in smaller crown facets and little to no fire).