Final answer:
An included diamond crystal that reaches the surface of a cut diamond is called an inclusion, and inclusions can affect a diamond's sparkle by interrupting its internal light reflections.
Step-by-step explanation:
An included diamond crystal that reaches the surface of a cut diamond is called an inclusion. The main purpose of the facets on a diamond is to promote total internal reflection, which occurs due to the diamond's high index of refraction relative to air. The diamond's critical angle is a mere 24.4°, meaning that when light enters the diamond, it is internally reflected multiple times, intensifying the light at specific points where it can exit, leading to the characteristic sparkle of the gemstone. Good quality diamonds are valued for their clarity, which allows for maximum internal reflection and accentuates their shine. Inclusions can interfere with the process of internal reflection, affecting the diamond's brilliance.