WoWDB

This shard creates a one-time use portal to Darnassus.  It is grown on your farm from an Unstable Portal Shard.  Which city your shard will grow into is random, although it will always be for a home faction city.  The shard "seeds" are sold by Barnaby Fletcher, the "Washed Up Mage" hanging around Halfhil market square for under 3g a piece.

These portal shards were added to the game in patch 5.1.