Reflection
Reflection
How light bouncing from a polished surface builds the lustre and brilliance gemmologists assess by eye
Reflection is the return of light from a surface without penetration into the underlying material. It is governed by a simple geometric law — the angle of incidence equals the angle of reflection, both measured from the normal to the surface — and it occurs at every boundary where two media of differing refractive indices meet. In gemmology that boundary is overwhelmingly air-against-stone, and the proportion of light reflected versus refracted into the stone is the foundation on which lustre, brilliance, and the grader's eye for surface quality are built.
Specular and diffuse reflection
Two regimes matter. Specular reflection occurs at smooth, optically flat surfaces and produces a coherent, mirror-like image: the polished facet of a faceted gem, the burnished culet of a wax model, the surface of still water. Diffuse reflection occurs at rough surfaces and scatters light in many directions; the matte finish of a sand-blasted metal or a poorly polished facet returns light without forming an image. Most real gem surfaces sit somewhere on the spectrum between the two extremes, and the cutter's polishing technique determines where on that spectrum the finished facet falls.
Refractive index and the proportion of light reflected
The Fresnel equations describe how much incident light a smooth surface reflects at a given angle. At normal incidence — light striking the surface perpendicular — the reflectance depends on the contrast between the refractive indices of the two media. Diamond, with a refractive index of about 2.42 against air at 1.00, reflects approximately 17 per cent of the light striking its surface at normal incidence. Quartz at 1.55 reflects only about 4.7 per cent. This is the physical reason that diamond appears to sparkle so much more vigorously than rock crystal of the same cut: a much larger fraction of incident light is returned as surface brilliance before any refraction or internal reflection plays a part.
As the angle of incidence increases away from the normal, the reflectance rises for both media, eventually reaching unity at grazing angles. The phenomenon is exploited in the refractometer, where the critical angle for total internal reflection inside a high-RI hemicylinder is read directly off a calibrated scale.
Reflection in cut and lustre
A faceted gem behaves as a system of internal mirrors. Light entering through the crown is refracted into the stone, internally reflected from one or more pavilion facets, and refracted back out through the crown to the eye. Whether each pavilion facet reflects efficiently depends on whether the light meets it above the critical angle for total internal reflection. Brilliant cuts in diamond are designed so that the great majority of incident light meets the pavilion facets above the critical angle and is returned to the viewer as crown brilliance. A pavilion cut too shallow or too deep leaks light through the back of the stone and the result is the dead-centred or fish-eye effect graders flag in poorly cut diamond.
For coloured gems, the same logic applies but the optimisation balances brilliance against the depth of colour the cutter wishes to display. Sapphire, with refractive indices around 1.762 to 1.770, supports a wide range of cutting styles; emerald, softer and more included, is conventionally cut with shallower step pavilions that trade some brilliance for a calmer, more even return of body colour.
Lustre and surface assessment
Lustre is the qualitative description of how a surface reflects light. Adamantine lustre, the highest, is the diamond-like brilliance also seen in zircon and demantoid garnet. Vitreous lustre, glass-like and the most common in gemstones, characterises corundum, beryl, and quartz. Resinous, waxy, silky, pearly, and metallic lustres describe progressively less reflective surfaces. The grader assesses lustre by tilting the stone under a single point source and judging the sharpness and coherence of the returned reflection; a sharp, well-defined reflection indicates a well-polished surface and high lustre, while a hazy or stretched reflection indicates surface damage, polishing residue, or an inherently lower-lustre material.
In the trade
Surface reflection is the first thing the experienced eye reads when a stone is presented across a desk. A lively reflection of overhead light off the table facet signals a well-polished crown and a healthy refractive index; a dulled or scattered reflection signals the opposite. Cleaners and polishers pay close attention to maintaining the reflective integrity of the table and crown facets because that is where the buyer's eye lands first. Any treatment, coating, or resurfacing that alters surface reflection — such as the cobalt or titanium coatings sometimes applied to enhance topaz colour — must be disclosed under AGTA and CIBJO rules.