What are Youmu Stones used for? In Kyoukai no Kanata, the hunters make a living out of selling Youmu stones, allegedly the souls of the defeated youmu. It goes even to the extent of a school teacher taking a leave to hunt during a particular event.

What is the use for those youmu stones after they are sold? Why are they valuable to others?



What I am looking for is some source material reference from other media (I only watched the anime).
I will build on FatalSleep's answer:


  The similarity among all youmu is their tendency to manifest an ore-like stone that contains the powers of that youmu when slain.


In the anime, it is shown that Sakura Inami can use Youmu stones to empower "her" spear-like weapon.

Given
(1) that the stones have the powers of the youmu and
(2) this use of the stones shown on screen,
we can infer that the stones could be used to empower/create magical items.

The barrier sticks, the charms and several other magical artifacts from the Kyoukai no Kanata universe could be made using the power from those stones.

As magical ingredients, youmu stones would be very valuable and highly sought after. This would justify the Ikaishi income, the wealth of some Ikaishi families and the economy of the Youmu stones market.