Wailing Stone

The "Wailing Stone" is an artifact found by the Crystal Gems in the episode "Rose's Room". It appears that the Wailing Stone can send both Audio and Video, although it was not originally intended to receive video. It was used in the Rebellion, as confirmed by Pearl, to send messages back and forth. The Wailing Stone's abilities can be augmented so that it can interpret messages that are normally too advanced by integrating it into a television.

"Rose's Room"
The Crystal Gems found the Wailing Stone on one of their missions. When the stone is activated by pushing the button at the top, it emits a very loud ear-piercing sound that causes objects to break like Steven's television screen.

"The Message"
Lapis Lazuli used a Wailing Stone to send Steven a message about the Homeworld Gems coming to Earth to find Steven and the Crystal Gems. She couldn't use the wailing stone successfully, and the message had to be shown through video.

Trivia

 * In "The Message", the Crystal Gems receive a video message from the Wailing Stone.
 * Originally, the stones only played simple audio messages, but due to the technological advances at Homeworld (as stated by Lapis Lazuli in the same episode), the frequency couldn't be translated to audio, because it was video.
 * Every single Wailing Stone on Earth belongs to the Crystal Gems, but there are other Wailing Stones in Homeworld.
 * The sound effects for the wailing stone were made by Rebecca Sugar playing on a modified Omnichord and Tony playing on effect pedals.
 * It has been shown that Wailing Stones can magnify the voices of humans and Gems, as unexpectedly demonstrated by Steven in "The Message."
 * It is said by Pearl that they used wailing stones during the rebellion, which is most likely the battle between Homeworld Gems and the Crystal Gems.