Now, the statistics that make a gemstone 'fiery' with a great deal of sparkle are refractive index and dispersion. Diamonds and Zircon have a similar dispersion which is the breaking of 'white' light into the seperate colors (blue, red, etc). They also have a close refractive index which dictates at what angle light can enter the stone and bounce around to create 'brilliance'. Finally, you can see that Zircon is birefringent which means it can split a single light ray into two inside the stone. This creates twice as much 'play' when the stone is moved around in the light. The only thing that saves a diamond is that it's extremly durable, and has a slightly higher refractive index (not to mention marketing).
Overall you can see here that the Zircon is considered a brilliant, lovely gemstone!