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A strangely tuned drive that produces a greenish contrail under boost.
Permanently upgrades an equipped Rare Sparrow to improve speed and durability.
― In-game description

An "Emerald Coil" increases both speed and durability of a Guardian's equipped rare Sparrow and turns the boost contrail green. The upgrade is a one-use consumable item that transforms the equipped rare Sparrow into a legendary Sparrow. It can be purchased from Xûr.