What are Alter Stones?

Alter Stones are items that can be equipped onto your weapon to help increase your stats.
You just have to have an Alter Stone in your inventory and double click on it. A menu will pop up and you have to click on your weapon. Think of these as sort of like Cards from Ragnarok Online 1 (If you played the game).

Select the slot that you would like to put your stone in and click on the top button when it ask you if would like to put the stone in. The stones should then be put onto your weapon and you would get the stats.

However, don't be afraid, you can replace an new stone over a old stone.Also, don't forget that you have to make sure that you have the right weapon level.
As your weapon levels, you would unlock more slots for your alter stones.
The max slots for the stone is 7 and you would unlock it when your weapon level reaches 20.