The symmetry of a diamond refers to its facets' alignment and proper positioning. A good artisan will expertly place each facet in appropriate proportions that complement the surrounding ones. The facets should be positioned with expert precision because misaligned facet junctions make a diamond appear uneven. Furthermore, the symmetric proportions of a well-cut diamond deter excessive carat weight from hiding in the profile, making a stone seem right for its weight.
The alignment of the facets also determines a stone's interaction with light. Some light that strikes a stone immediately reflects as surface glare. While a portion of the light that enters the diamond escapes through the bottom, having no visual effect on an observer. A too shallow or too deep cut allows light to leak out through the crown or the diamond's internal walls, making many facets appear dark or dull, and the overall appearance is less brilliant and sparkly. Properly positioned facets maximize the amount of light that bounces back (reflects) from the tabletop or bends (refracts) and off the internal walls into an observer's eye.
The observable effects of the internal and external interactions with white light constitute a stone's brilliance. Scintillation will result from an equal balance of dark and light areas, forming a crisp, clear pattern with no distracting dark outlines as the light exits a stone. Fire results from the light being dispersed into a rainbow of spectral colors. If the light source, diamond, or the observer moves, spots of light that flash (sparkle) will be noticeable.