DIAMOND AND LIGHT.

In the long run, the diamond beauty is determined by both optical properties of diamond as a mineral and optical effects obtained as a result of cutting and polishing. The human eye perceives diamond as a combination of its surface and interior optical properties: scintillation, brilliance, and fire.
Scintillatio
Graphic5.GIF (28536 bytes)Scintillation is the light reflected from the stone surface. We can observe luster (and see the stone surface) only due to some light reflected without penetrating into a stone
Reflecting power, which determines the luster intensity, is a physical property. Physical properties of minerals are usually described qualitatively. For instance, different minerals are characterized by metallic, glassy, brilliant, pearl, or another kind of luster. The brilliant luster is very high, i.e. minerals with a brilliant luster (including diamond) have a high reflective power. Luster, which is seen on naturals and shears of a crystal, looks most advantageous on the flat polished surfaces reflecting the whole bundle of incident light in the same direction. Thus, it seems that the more polished a stone, the higher its luster. Luster also depends on the angle of light incidence on the stone surface (reflection factor is 17% for a beam incident on the surface at a right angle). At last, clear diamond has a higher luster than a that with a contaminated surface.

Light refraction

occurs at the air-stone interface as a light beam enters a stone or comes out of it. The refractive index of the air is assumed to be unity, while the refractive index of diamond is equal to 2.42, one of the highest values among transparent minerals. It is the great difference in refractive index between air and diamond that causes a significant change of the direction of a light beam at the air-diamond interface (Fig. 5). The light is refracted so that its angle of incidence in the air is larger than its angle of refraction in diamond (relative to the perpendicular to the air-diamond interface). Therefore, the light beam is more deflected from the perpendicular direction as it comes out of a diamond crystal. From a certain angle, the light passing through the diamond to the air-diamond interface is completely reflected inside the diamond (Fig. 6). Such an angle value is defined as critical. The critical angle for diamond is equal to 24,5 degrees. In a cut diamond with properly verified proportions and angles, the light entering through the crown facets twice undergoes total reflection at the pavilion facets (according to the law of total internal reflection) and comes out through the crown (into the eye of a spectator). Due to this property, diamond does not let light pass through, unlike most imitations. As the light refracted inside the diamond comes out, it is mixed with the reflected light. In the strict sense, luster is a mixture of refracted and reflected beams. The fraction of light returning to a spectator's eye compared to the amount of light striking the stone surface is sometimes described as brilliance.

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Dispersion
Graphic3.GIF (12204 bytes)Dispersion is the ability of a stone to split a light beam up into spectrum. It is practically manifested as red, green, blue, and other colored beams coming out of a stone. This property is also constant for every individual substance and is determined by the difference in refractive index between the beams of different colors (Fig. 7), with the refractive index for violet is higher than that for red. The dispersion value for diamond is equal to 0.044, which is one of the highest values among minerals. Any white beam passing through a diamond is split up into spectral constituents; however, the dispersion can be visible only if some of the colored beams come out of the stone, while others are reflected inside. Such condition is not fulfilled for every constituent beam; hence, most of the beams coming out of a cut diamond are white, and only a few are colored. The longer the way the light passes in a cut diamond as a result of repeated reflections, the more diverged become the colored beams, and, therefore, the more intense the dispersion. As a consequence, a large diamond shows a higher life than does a small one.
Color of stone

Color is generally of primary importance for gemstones, but most of diamonds are colorless or near colorless. Hence, diamonds are primarily valued for their scintillation, brilliance, and fire, which are determined by the three properties described above: reflection, refraction, and dispersion. To describe these properties in combination, the term brilliance is often used, which denotes the overall optical image of a diamond. One can say that a stone is characterized by brilliance and color.

Color of stone is a property determined by selective light absorption. The ideal diamond is absolutely colorless; however, an actual specimen always contains microimpurities and structural defects, which absorb a part of the light passing through the stone. For example, the yellow color, which is typical of most diamonds, appears in the specimens that mostly absorb the blue and green constituents of the light spectrum (Fig. 8). Rare colored diamonds has a different absorption character. Diamonds containing few color centers have a tint rather than a color. On the other hand, intense absorption affects the transparency of a stone, and dark stones are perceived as translucent.

The ultimate impression of a diamond is formed by reflection, refraction, dispersion, and absorption, which determine its : scintillation, brilliance, fire, and color. Perception of the optical properties of diamond by human's eye can be described by the following scheme: