The consecration seed is an item used to perform an elven consecration on the tomb of Glarial in the Roving Elves quest. The item can be obtained by killing one of the Moss Guardians in Glarial's Tomb. Eluned can enchant the seed to prepare it for the consecration. If you lose the seed during the quest, just go back to Eluned and you will get a new consecration seed.
For an exhaustive list of all known sources for this item, see here
The player plants the consecration seed, and a crystal tree grows.