Surface-to-air rocks

Surface-to-Air rocks, otherwise known as Surface-to-Surface rocks due to their alternate use, are projectiles used by the Royal Earthbender Guards as their primary weapons. Usually launched from a courtyard directly outside the Earth Kingdom Palace, these are used both as offensive and defensive armaments.

The primary purpose of Surface-to-Air rocks is to prevent intruders from entering the royal palace either by foot or from the air. They are large rounded trapezoids prepared by the dozens. When launched, these rocks are flipped so that the small jutted side impacts the target first. This is advantageous as the rock's positioning makes it less likely for intruders to avoid the attack.

Once possible threats to the Earth King's safety are spotted, several of these rocks are lined up and fired in succession, so as to effectively prevent invaders from bypassing the palace's defenses.

History
After arriving to Ba Sing Se, Team Avatar sets off to find the Earth King and recruit his help in a planned invasion of the Fire Nation Capital. However, while attempting to do so, the team faces a variety of problems, mainly the encumbrance of the city's strict policies and the hindrance brought upon by Long Feng and his fleet of Dai Li Agents.

In a last-ditch effort to notify Earth King Kuei of an upcoming solar eclipse that would render Firebenders powerless, Team Avatar decides to breach the palace by force. Immediately after entering the stronghold's perimeters, Royal Earthbender Guards target Appa and fire earthen trapezoids toward him. Aang is easily able to destroy the rocks upon contact through the use of Earthbending and Team Avatar lands with relative ease.

After contacting the ground, Toph displays her Earthbending prowess, facilely defeating large numbers of Earthbender guards with nearly no effort. Katara also proves herself to be a capable combatant, as she easily defeats several lines of troops while pushing forward to the palace doors.

Eventually, the team reaches the Earth King's Throne Room, and after presenting enough evidence, they are able to show him that he has been subject to propaganda by his supposedly "loyal" adviser. Team Avatar successfully enlists the Earth King's support in the Invasion of the Fire Nation, but is ultimately unable to use it following the fall of Ba Sing Se.

Trivia

 * Surface-to-Air rocks and their alternate use of Surface-to-Surface rocks share their namesakes with real-world weapons; Surface-to-Air missiles and Surface-to-Surface missiles (SAMs and SSMs respectively).