Extension stones can be used on ANY High Grade items in game, it can be last forever (if you have ext. stones) :D
- High Grade item will be 7 - 14 days as its expiry date once you unseal it.
- Each Ext. stone, it extent our HG item by 10 Real Life days on the expiry date. Example: a HG gun was unsealed on Year   M  D , and expiry date on Y  M  D, 14days. when we use 1 ext stone on it, it become expriry date Y M D.
- Extension stone can be used on HG item when it' Sealed or Unsealed. :D It will be added the 10 days into the HG itself.
As below, put the gun and ext stone. Click the button Expiry Extension. That's it. Currently, it is 100% success on Extension.
How to play Extension stone on HG item:
Hope this post will be helpful for who want to know more about Extension stone.