Final answer:
Gemstones are carved 'en cabochon' rather than faceted when they exhibit optical phenomena that are enhanced by a smooth surface, or when they lack clarity and have inclusions, as this cut can hide imperfections. Conversely, stones like diamonds are faceted to maximize sparkle through total internal reflection.
Step-by-step explanation:
Some gemstones are more commonly carved or cut 'en cabochon' rather than faceted due to various factors including the presence of special optical properties, the overall clarity, or the nature of the inclusions within the stones. Stones that display phenomena like asterism (star effect) or chatoyancy (cat's eye effect), which can be enhanced by a smooth, rounded surface, are typically cut en cabochon. Gemstones that lack the necessary clarity or have numerous inclusions are also better suited to this style of cutting, as it can hide imperfections better than faceting. Conversely, stones such as diamonds require faceting to enhance their sparkle through total internal reflection, which is optimized by cutting facets at precise angles to maximize light refraction and reflection. This is because diamonds, and other gems with a high index of refraction, have a critical angle that confines the light within the stone, allowing it to make multiple internal reflections before exiting, resulting in a sparkling effect. However, stones that do not possess such optical properties or clarity are not enhanced by faceting and are therefore typically not faceted.