Why Is The Color Of Emeralds From Colombia Considered The Finest?

It's the pyrite. Emerald color gets blunted by the presence of iron atoms (even one per million) in the emerald crystal. The presence of pyrite in the Colombian emeralds geologic environment sweeps up all of the iron atoms to form pyrite crystals, leaving the emerald free from gray or yellowish undertones, allowing its natural fluorescence to charm the eye.

Useful link:
http://www.emeraldmine.com