Small pendant (or bracelet) shaped device used by the Wraith to transmit information about enemies to relay devices, which then transmit the intelligence to a nearby Wraith ship. The pendant versions have been easily mistaken for other things, such as simple jewelry.

Tagan gave his daughter Teyla Emmagan such a gift, but when she was a child she lost it in the old caves of Athos where she was playing. Major John Sheppard later found the device. By simply touching it he set it off, quietly sending a signal to the Wraith, inviting an approach.

KEY EPISODES

Rising, Part 1 - Sheppard finds Teyla's old necklace and returns the property to her, not realizing he is likely responsible for the attack the Athosians are about to receive.
Suspicion - After falling under attack in five out of nine missions Sergeant Bates orders Dr. McKay to search Teyla's belongings, and finds the Wraith transmitter concealed in the form of her necklace.
Childhood's End - Sheppard finds an alternate version of the Wraith transmitter technology in a memorial on M7G-677.