A doubleering is rainbow-like in appearance. Many confuse this with the flash effect of fracture filling.

Do not confuse a doubleering with fracture filling. A doubleering is an inclusion, probably a feather, that takes on a rainbow-like appearance. Often it is open to the surface, but not always. The obvious difference between a doubleering and the flash effect of fracture filling, is the color consistency. A doubleering displays a mixture of colors, like a rainbow, whereas fracture filling shows one distinct and consistent color in its area of the diamond. (A blue flash can occur in one area, while a green flash may occur in a separate area in the same stone.) Multi-colors (or one color) in different areas of a diamond is indicative of fracture filling, whereas multi-colors (rainbow-like), intermingled in the same area, is indicative of a doubleering (nonfracture filling).