For clarity enhancement, the material used to fill a gem's fractures must have what property?

When it comes to clarity enhancement in gemstones, the filling material used to address fractures must ideally possess a matching refractive index to the gem itself. This is vital because the refractive index determines how light bends as it passes through the gemstone and its inclusions. If the filling material has a refractive index that closely aligns with that of the gemstone, it minimizes visual distortion and makes the filled fractures less noticeable.

This matching property helps maintain the gem's overall optical transparency and brilliance, allowing light to pass through the gemstone uniformly, rather than scattering or reflecting in an unintended manner at the boundaries between the filler and the gem. This ensures that the enhancement effectively improves the appearance without creating additional visual complications.

Other options may contribute in their respective contexts, but they do not address the optical integrity as directly as the refractive index does. For instance, matching color can help visually disguise fractures, but it doesn’t solve the optical challenges posed by the differences in how light interacts with materials of different refractive indices.