Final answer:
A diamond's cut and clarity tremendously affect its brilliance due to total internal reflection and its high index of refraction; slight variations determine if the diamond becomes a gem or a drill bit.
Step-by-step explanation:
Slight variations in a diamond's cut or clarity can have a significant impact on its use as either a fabulous gem or a functional drill bit. The total internal reflection of light and the large index of refraction are critical factors that contribute to a diamond's sparkle. The diamond's critical angle with air is merely 24.4°, making it difficult for light to escape, allowing for multiple internal reflections which lead to the diamond's brilliance. Facets in diamonds are crafted to ensure light is reflected and exits in a concentrated manner, enhancing the sparkle. Additionally, colored diamonds contain structural defects or foreign materials that contribute to their hue, unlike clear diamonds which usually appear nearly colorless and sparkle due to dispersion of light.