Emerald Radar

Emerald Radar is a gimmick used by Knuckles the Echidna and Rouge the Bat in Sonic Adventure and its sequel, Sonic Adventure 2. When using this, the user can search for Master Emerald shards or Chaos Emeralds.

Description
In both Sonic Adventure and Sonic Adventure 2, the main gameplay style for Knuckles and Rouge are colloquially dubbed "Emerald Stages," mainly because the main objective is collecting Master Emerald shards, or, in Rouge's case, keys or even Chaos Emeralds. Three icons representing Master Emerald shards/Chaos Emeralds are below the screen, in which if a nearby gem is close by, the emerald icon below will begin to turn green. Moving closer to the emerald will turn the green icon to yellow. If very near to the emerald, the icon will turn red.

In Sonic Adventure, Knuckles can search for any emerald at any time. However, in Sonic Adventure 2, both Knuckles and Rouge are limited to finding the first emerald icon at a time, then moving on to the other emeralds. This is very infuriating to most players, since sometimes players will be able to find an emerald but the Emerald Radar will not flash because it is one of the emeralds that must be collected later.

Archie Comics
The Emerald Radar also appears in the Sonic the Hedgehog and its spin-off comic series' published by Archie Comics. It is a natural trait of Knuckles, essentially a sixth sense to the echidna. It is among one of the many move sets Knuckles has learned in the Post-Super Genesis Wave timeline.

Knuckles first used the Emerald Radar in Sonic Universe #63 to help the Chaotix find a Chaos Emerald upon Sally Acorn's request in order to restore the planet after the beginning of the Shattered World Crisis. Using his Emerald Radar, Knuckles and the Chaotix would travel from Pumpkin Hill Zone to Aquatic Mine Zone to find the nearby Chaos Emerald before rival team Hooligans do first. Fortunately, Knuckles and the Chaotix had found a Chaos Emerald, but upon receiving it, they found Chip holding on to it.