It's a type of obstacle in Sword of Mana found in several dungeons. It consists of a grey stone structure in the shape of a elongated hexagonal bifrustum, containing strange markings that glow around it and a hexagon on top.
Like the Metaball, they are immobile guarding a place, only when the character gets close to it, they launches an attack, preventing the character's passage, but in their case, the attack consists of a magical attack that will depend on the type of seal. Also, despite giving experience points like Metaballs, they are not considered monsters, since they are not included in the bestiary.
According to Popoi's Note about they: There are 8 types of stone seal: Light, Dark, Moon, Fire, Water, Wood, Wind and Earth. Each stone inflicts damage based on its type. A seal's type can be determined by the color or its hexball. To destroy them, physical attacks don't work, the protagonist need to use the magic of the mana spirit corresponding to them, in this case: