ASS #7 Key Terms & Concepts PDF

ASS #7 Key Terms Adamantine—The highest degree of luster possible in a transparent material. Angle of Incidence—The angle at which light hits a surface or boundary, measured between the normal and the incident light. Angle of Reflection—The angle at which light returns from a surface...

ASS #7 Key Terms Adamantine—The highest degree of luster possible in a transparent material. Angle of Incidence—The angle at which light hits a surface or boundary, measured between the normal and the incident light. Angle of Reflection—The angle at which light returns from a surface or boundary, measured between the normal and the reflected light. Angle of Refraction—The angle at which light bends when it travels from one material to another, measured between the refracted light ray and the normal. Brightness—The effect of all the diamond's internal and external reflections of white light. Critical Angle—The angle of incidence at which the angle of refraction reaches 90 degrees and total internal reflection begins to occur. Dispersion—The optical phenomenon of white light separating into spectral colors. Luster—The appearance of a material’s surface in reflected light. Normal—An imaginary line perpendicular to the surface of a boundary. Optical Density—The ability of a medium or material to slow down the light that travels in it. Path Length—The total distance a light ray travels in a gemstone. Pattern—The relative size, arrangement, and contrast of bright and dark areas when viewed face-up. Polish—The overall condition of the facet surfaces of a finished diamond. Reflection—The light returned when it hits a surface or a boundary between two different materials. Refraction—The change in speed and possible change in direction of light as it travels from one material to another. Refractive Index—A measure of the change in speed and direction as light travels from the air to a certain material. Symmetry—The exactness of a finished gem’s shape and facet placement. Total Internal Reflection—Complete reflection of a light ray within a material. Key Concepts The angle of incidence and the angle of reflection are always equal. In gemology, the refractive index (RI) of air is always assumed to be 1. All gem materials have RIs higher than 1. Light travels more slowly in materials of higher optical density (higher RI) than in materials of lower optical density (lower RI). The refracted light bends toward the normal when light travels from a less refractive material to a more refractive material. The refracted light bends away from the normal when light travels from a more refractive material to a less refractive material. Materials with higher RIs have smaller critical angles than materials with lower RIs. The critical angle of diamond is 24.5 degrees. Diamond’s small critical angle limits the opportunity for light to exit the stone. Dispersion is measured by the RI difference between the violet and red light rays in a material. Dispersion is an optical property that varies by material. The greater distance light travels in a gemstone, the more absorption occurs. Diamond’s brightness is related to the amount of light reflected back to the observer. A diamond’s face-up pattern reveals nearly all of the cutter’s fashioning choices. A diamond’s display of dispersion is described as “fire” by diamond professionals. The best way to see a diamond’s fire is to look at the stone face-up while rocking and tilting it at the same time. In colored diamonds, more absorption causes deeper color, and less absorption causes lighter color. Cutting can alter a diamond’s color appearance by changing the length of the path of light in the diamond. The GIA Cut Grading System is based on both scientific research and observation. Each cut grade includes a range of proportion combinations and a variety of appearances. The GIA Cut Grading System applies to round brilliant diamonds in all clarities across the D-to-Z color range. Each cut grade is a combined evaluation of face-up appearance, design, and craftsmanship. Variations in lighting and surroundings can affect a diamond’s appearance. Diffused fluorescent lighting helps you evaluate a diamond’s brightness and pattern, while spot lighting helps you evaluate its fire.