A handful of powdered bone.
How to Obtain
Bone Meal can be found as a random drop from mobs. (Notably undead / skeletons in for instance the Serbule Crypt) They can also be found in "skeletons" that stick out from the ground near the crypt. This was more successful than as a random drop.
One of the uses for Bone Meal is to create Bottle of Fertilizer for Gardening. After gaining favor with Therese to Friends, she will also teach you a recipe how to make Fertilizer from Bone Meal.
Note: to make fertilizer, you may be better of using Strange Dirt. It only takes 1 Strange Dirt to make fertilizer, as apposed to the 9 Bone Meal.