How does fracture filling reduce visibility of fractures and cleavages?

• A. By using a low refractive index resin.
• B. By applying laser light to fracture surfaces.
• C. By using a high-RI glass filler.
• D. By compressing the crack with a metal insert.

Answer: (C)

Fracture filling works by creating an optical bridge across the crack. The filler material—often glass or resin with a relatively high refractive index—is introduced into the fracture and clears it up so light can pass through more uniformly. When the filler’s refractive index is closer to that of diamond, the boundary between the crack and the surrounding crystal causes much less light scattering and internal reflection. The result is that the fracture is much less visible to the eye, especially under magnification, because light travels through the filled area with similar optical behavior as the surrounding gemstone.

A low-refractive-index resin would leave a larger mismatch at the crack, making the fracture more noticeable. Laser treatment or inserting a metal piece would not create a continuous, optically similar path for light through the crack, so they don’t effectively hide the fracture. Using a high-refractive-index filler aligns with the goal of optical match and fracture concealment.