You were told of a long played-out mithril-mine within which an ancient smelting-forge lies. Within it, you should be able to lay the basis for the quality you seek. However, a vicious band of Orcs has enslaved their own goblin brethren to eke out the remining tailings from the mine, making it difficult to reach the forge.
You must find enough fragments of mithril ore to make proper use of the smelting-forge for the quality. Hopefully, the goblin-slaves have managed to collect enough of the ore for your needs...though they are unlikely to part with it willingly.
Use the ancient forge and the mitrhil ore-fragments you have collected to empower your infused garnet.