FJORD (FORGE LANDS)
1) The stone is hidden in Vulgrim's hideout, located in the south of the Fjord (screenshot 1). When you get there, do some climbing and use two portals to get to the upper room with shadowbombs. More information on how to do it are detailed in the Additional Locations chapter, in the Forge Lands - Other locations section.
FOUNDRY (FORGE LANDS)
1) The stone can be found in one of the first areas of the Foundry. Stop where you reach the place where large cauldrons are being transported (screenshot 1). Wait until there appears a cauldron with the stone attached to it (screenshot 2) and shoot it before it leaves you fire range.