The half a rock is a quest item used in the Mountain Daughter quest. The teardrop-shaped rock is obtained by using a pickaxe on the Ancient Rock in a tent in the Mountain Camp. The player must give the rock to Brundt the Chieftain in Rellekka to obtain a safety guarantee for Svidi.
Interestingly, when the player tries to obtain two pieces of the rock using the drop trick, they receive a message stating, "While two halves make one whole, it's better to just have half a rock." However, a player may obtain two pieces by using the drop trick and casting telekinetic grab on one of the halves. Strangely, using both halves together in a Player's inventory will not create a whole rock.