What is a flat, triangular twinned diamond crystal called?

A flat, triangular twinned diamond crystal is known as a macle. Macles are specific formations of diamond crystals that exhibit a twin growth habit, where two individual crystals merge together along a specific crystallographic direction. This unique structure gives macles their characteristic flat and triangular appearance, distinguishing them from other types of diamond crystal forms.

In the context of gemology, identifying macles is important as they can influence the gemstone's value and appearance. They are often cut to enhance their natural features and to create unique gemstones.

Other terms such as crystal, refractive, and facet refer to different concepts in gemology. "Crystal" is a broad term that applies to all forms of crystalline solids, while "refractive" pertains to the way light bends as it passes through a medium, which is an important property in evaluating gemstones, but not a term for a specific crystal formation. "Facet" refers to the flat surfaces cut onto gemstones to enhance their sparkle and brilliance, not to a specific type of crystal structure. Therefore, the term macle is the correct choice as it specifically identifies the unique twin crystal formation of diamonds.