A Harmonised coal rock is a special rock that can appear in the Trahaearn Clan district's mining area in Prifddinas while the Voice of Seren is active for the clan. The rocks have an endless supply, but collapse after a fixed amount of time, however more can appear. The mining experience is the same as the regular rocks; however, because they can only appear with the Voice of Seren active, there will always be an experience boost active when they appear.
Multiple players can mine from the harmonised coal rocks and all will receive coal. Four of the eight coal rocks can harmonise at one time.