Emerald Detector

The Emerald Detector (which is the universal term for it) is a machine that appears in the Sonic the Hedgehog series. It is a portable device, able to track and locate emeralds infused with chaos energy, such as the Chaos Emeralds, the Master Emerald and Emerald Shards. During the ongoing hunts for such emeralds, the Emerald Detector is a valuable device and has been used by heroes and villains alike.

Sonic the Hedgehog 3
The first Emerald Detector to appear was mentioned in Sonic the Hedgehog 3, where it was called the Jewel Radar (ュエルレーダー). In this game, it was developed by Tails, where it detected an reaction from the Master Emerald after the Death Egg landed on Angel Island, prompting Sonic and Tails to head out and investigate.

Sonic Riders
In Sonic Riders, an Emerald Detector was used by Team Sonic to find the Chaos Emeralds. In this game, it resembles a stopwatch with a radar screen, a button display on the bottom and a small antenna on top. Its screen can display the surroundings in 3D, allowing for better location of the Emeralds.

Team Sonic used the Emerald Detector to track a Chaos Emerald to Metal City. As they experienced interferences, the Emerald Detector was able to pinpoint an Emerald in the possession of the Babylon Rogues.

Sonic Rivals 2
In Sonic Rivals 2, another Emerald Detector appeared, though it was not seen by the player. In this game, it was made by Dr. Eggman Nega and was powered by Rings. The Master Emerald, which Nega had stolen, was inserted into the Emerald Detector, though it is not known what purpose it served.

Presumably used by Nega to find the Chaos Emeralds for his plans with the Ifrit, the Emerald Detector was taken from him by Knuckles and Rouge who used it to find Emeralds as well; Knuckles sought the Master Emerald while Rouge looked for the Chaos Emeralds. At the end of their hunt, where Rouge found the Chaos Emeralds thanks to the Emerald Detector, Knuckles discovered the Master Emerald inside it, much to his dismay.

Sonic Runners
In Sonic Runners, an Emerald Detector, also called a Master Emerald detector, was used by Dr. Eggman to track down the Emerald Shards of the Master Emerald. In this game, the device is a rectangular red pad with a round green radar screen, buttons on the edges and an antenna on top.

Early on, the Emerald Detector was used by Dr. Eggman to track Team Sonic's movements, who was also hunting Emerald Shards, but when Eggman confronted them, he lost the Emerald Detector to the heroes who used it themselves to find the remaining Emerald Shards.

Trivia

 * The Emerald Detector bears a few key similarities to the Dragon Radar from the manga Dragon Ball:
 * They are both used to track down the prominent objects in their respective series; the Dragon Radar locates the Dragon Balls, and the Emerald Detector locates emeralds with Chaos energy.
 * The Emerald Detector's design is very similar to the Dragon Radar; in Sonic Riders its design looked nearly identical to the Dragon Radar, in Sonic Rivals 2 it would beep when near the sought out objects, and in Sonic Rivals its radar display was nearly identical to the Dragon Radar's.