Brightness in gemstones is an attribute of visual perception in which a gemstone appears to be radiating light. But few gems actually radiate light and what we see is reflected light. Brightness in the context of gemstones is a subjective attribute and there is no way to quantify it.
The "fire" of Sphene derives from its very high dispersion combined with a high index of refraction. Gems with a high refractive index, and a high dispersion will usually elicit an impression of greater brightness. A diamond’s refractive index is 2.417 and its dispersion is .044. Sphene’s refractive index is 1.900 – 2.034 and its dispersion is .051 so it is arguably a top range brilliancy contender if brilliance could actually quantified.
The dispersion of sphene exceeds that of diamonds and shows flashes of gold, yellow, red and green that diamond cannot. And due to this high dispersion and pleochrism, a well cut stone can reveal a stunning brilliance that will vary in appearance as a function of the light source and the angle of view. Generally it is green, with just about every other color of the rainbow mixed in somewhere.