Final answer:
The top-most facet of a gemstone is known as the table. The table, along with other facets, helps create the sparkle in gemstones through total internal reflection and high refractive indices.
Step-by-step explanation:
The top-most facet of a gemstone is called the table. Gemstones like diamonds are crafted with various facets to create their stunning sparkle. Specifically, the facets are cut in such a way to take advantage of the property of total internal reflection, which, when coupled with the gemstone's large index of refraction, results in the characteristic sparkle that diamonds are known for. The critical angle for a diamond-to-air surface is approximately 24.4°, meaning that when light enters the diamond, it tends to get reflected internally rather than passing back out of the diamond. The table is one of the larger facets that play a role in the refractive and reflective properties of gemstones, which contributes to their unique glittering effect.