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.
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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. |
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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. 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. 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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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.
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:
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