Cambodian Blue Zircon is considered the 'King of Zircons' due to the medium to strong blues they consistently produce. 

There is no naturally blue zircon found anywhere on earth. All blue zircon has been heated in an oxygen reduced environment to turn an often dark brown zircon blue. And not all zircons turn blue, only a couple locations like Nigeria, Vietnam and Mali mine a zircon that can be heated blue. Nigerian and Vietnam zircons heat to a pale blue while the Mali material has produced some desirable blues

3.289 grams / 16.445 carats / 18.9 x 11.3 x 8.2 mm / clean