Final answer:
The 4 C's of diamond selection are cut, color, clarity, and carat weight, not cost. High-quality diamonds sparkle with brilliant colors when illuminated, due to dispersion and total internal reflection within the carefully cut facets.
Step-by-step explanation:
The correct answer to the question, "Which one of these is NOT one of the 4 C's of selecting a diamond?" is c) cost. The four C's of selecting a diamond actually refer to cut, color, clarity, and carat weight. Cost is not one of the standard criteria, although it is, of course, a significant factor in purchasing a diamond.
A high-quality diamond can sparkle with flashes of brilliant color when illuminated by white light because of dispersion and total internal reflection. Dispersion occurs due to the different degrees of bending (or refraction) that different colors of light experience when passing through materials like diamonds. The diamond's high refractive index makes it so that when light enters the diamond, it has difficulty getting back out unless it meets a very shallow angle. The meticulously cut facets on a diamond are designed to maximize the number of internal reflections, enhancing the diamond's brilliance and allowing the light to exit in concentrated beams, giving rise to the sparkle we see. Additionally, the clear nature of a quality diamond ensures that light can travel freely and multiply reflect within the stone before emerging.