What is the flash effect in fracture-filled diamonds?

• A. A color change seen on the surface only
• B. A constant color observed under UV
• C. A color change seen during viewing at 90 degrees to the inclusion
• D. A flash of changing color seen in a fracture-filled diamond when viewed parallel to the filled inclusion and rocked under magnification

Answer: (D)

The flash effect is a dynamic color phenomenon caused by light traveling through a fracture that has been filled with a material of different optical properties than the host diamond. When the filled fracture is oriented parallel to the viewer’s line of sight and you rock the stone under magnification, you observe a quick change in color—a flash. This happens because light paths inside the fill and at the fracture boundaries create interference and dispersion that strongly depend on the viewing angle. The filled material often has a different refractive index than the surrounding diamond, so as you rotate and tilt, the wavelengths that combine constructively or destructively shift, producing that brief color burst. This effect is specific to the parallel orientation and magnified viewing; it’s not just a surface color change, nor a constant color under UV, and it isn’t observed in the 90-degree orientation in the same way.