What shape are the majority of facets in a brilliant cut gemstone?

In a brilliant cut gemstone, the majority of facets are triangular. This is primarily due to the design and purpose of the brilliant cut, which is to maximize light reflection and refraction, enhancing the gemstone's brilliance and sparkle. The triangular facets work effectively to create the right angles for optimal light play, allowing the stone to exhibit its best optical performance.

The structure of triangular facets allows for a greater number of them to be incorporated in the traditional round brilliant cut, contributing to the gem's overall light-return capabilities. This design choice is crucial for achieving the desirable qualities of brightness and fire, which are hallmarks of well-cut brilliant gemstones.

The fact that some shapes and facets are not primarily triangular, like square, rectangular, pentagonal, or hexagonal, is less effective in producing the same level of brilliance. As a result, these options do not accurately represent the design principles behind the brilliant cut, making the triangular facet option the correct choice.