Drakestone is a plot item in Dragon Age II.
Travel to the The Bone Pit, where you enter the Drakestone Mines to find five samples of drakestone.
Acquiring this item is part of the quest Justice (quest).
Drakestone's status as a yellow rock and its relation to dragons and fire could relate it to sulfur, a key ingredient in gunpowder. Considering the other mineral the player gathers for Anders is sela petrae, likely a reference to saltpeter, this is much more likely.