Final answer:
The correct answer is 'b. facets,' which are created when grinding the diamond gem on a spinning lap to generate the diamond's sparkle through total internal reflection.
Step-by-step explanation:
The diamonds we typically see today in engagement rings do not display their natural crystal faces but rather facets, which are made by grinding the gem on a spinning lap.
The total internal reflection and the large index of refraction are responsible for the diamond's sparkle. With a critical angle of only 24.4° for a diamond-to-air surface, light entering a diamond finds it difficult to exit. These facets are intricately placed to ensure that light is only able to escape from the diamond at certain points, which generates the much-admired sparkle.