Many faceters are familiar with bow ties. High fashion neckwear excepted, bow ties are generally not a good thing in the faceter's world. A commonly encountered pattern of bright and dark regions in stones suffering from this malady consists of opposed pairs of triangular regions sharing a common, central vertex. When these causes are understood and appreciated the faceter is better equipped to judge which gemstone designs are likely to suffer from bow ties, and why cutting them in materials with relatively low refractive indexes like quartz and beryl can aggravate that problem.
Gem materials exhibit characteristic refractive indexes which vary from species to species.
Light enters and exits gemstones through refraction. Depending on its incident angle with the surface of a facet and the refractive index of the material, light striking the facet will either reflect back off the surface of the facet, or refract and pass through it. Pavilion facets do their job by reflecting light which has entered the gemstone and is propagating through the gem material. If the geometry of the pavilion facets is such that light strikes them at an angle below the critical angle, it is refracted and leaks out of the pavilion, rather than being reflected and returned to the viewer through the crown of the gemstone. A very simple gemstone model can be employed with a ray tracer as a model to illustrate how light is leaked from the pavilion when its facet angles are too shallow or too steep.