Goibne Stone?

I was wondering what the exact purpose of the Goibne stone was. On the special upgrade page, it states directly .. "To perform a special upgrade you must have a weapon that has all 5 upgrades, as well as the jewel upgrade and have used the Goibne Stone on it."

But I have yet to see one, and I have already put on the first R-type Upgrade onto my Francisca. Will I need to find this stone to further upgrade it, or is it some other purpose entirely?

To simply clarify... I did not use the stone Goibne stone in any way. It was not necessary to put on the R-Type upgrade or any process leading up to it