You found a Map Stone Fragment! Return this to a Map Stone to unlock a new area map!
Map Stone Fragments are pieces of Map Stones that are found throughout Nibel. When a map stone fragment is placed into a map stone, a large portion of the map is revealed. Map stone fragments are scattered throughout Nibel and usually require some extra effort to collect.
The game ignores where a map stone fragment came from, so what portion of the map is revealed is determined solely by the map stone itself. For example, bringing a map stone fragment from the Sunken Glades and using it in the Forlorn Ruins will still reveal all of the Forlorn Ruins.