The Echo Gem is a Quest Item available in Golden Sun: Dark Dawn. It's desciption reads as 'An heirloom of Lady Hinechou's family.' It is available when the player reaches Tonfon and enters the palace. The Echo Gem is guarded heavily by one of Lady Hinechou's guards in a chest of drawers in her personal quarters. The Echo Gem comes with a pair, and her children Hou Ju and Ryu Kou have the other. Because her children are missing, she hopes the Echo Gem will in some way help her find if her children are still alive before Matthew's party manages to bring them back to Tonfon. The Echo Gem is obtained by using the Slap Psynergy on the gong in the palace, which alerts the people within that there is danger. Upon hearing the sound of the gong, everyone in Lady Hinechou's room rushes to surround and protect her from a potential attack, including the guard that protects the chest of drawers, which leaves you able to take the Echo Gem from the drawer. It begins to resonate as soon as it's in your possession, indicated by three wavy green lines next to Matthew's face icon on the map. It leads you into the direction of Hou Ju and Ryu Kou's Echo Gem and eventually leads the player to the Iceburg Outpost. Hou Ju uses her Echo Gem as a marker for Hou Zan's grave.
The Echo Gem can be sold for 2400 coins and can be bought back for 3200.