the stone—its luster. Diamond's adamantine ("diamond-like") luster is second only to metallic (i.e., that of metals); 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 (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. 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: 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 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