Infested Emeralds is a starter of range protection among the Arctic Zergs, giving new Zerg units working like Protoss observer and altering Ultralisks in more defensive way.

Since Arctic Zergs are mentally a strange case of Zerg literally worshipping the Protoss race, having infested people partly responsible for this phenomena, they needed a structure that would be of religious work. In the end, Infested Sapphires served that purpose.

(Zerg belongs to Blizzard)