Fourth Stone Crafter is located in the Sanctum near Impetrible Blacksmith.
Fourth Stones are crafting material needed to craft Legendary Items, Basic & Master Gems.
Each Stone requires a collection of items given from the npc. You will Need Every Stone for crafting so make them all.
Gem Master. The Gem Master Crafts basic and master gems of which are stat gems that slot into accessory and weapon items. basic gems give +2 - +5 to all stats, As Master Gems give +10 to all stats. Basic gems can only be slotted into accessory and Master Gems can be slotted in Weapon AND Accessory.
Etherdeocon is the Master Gem that can be slotted into your weapon.
Etherbradium is the Master Gem that can be slotted into your accessory.
To Craft the master gems you will need to make Etel Dust and a Beaker and a few other items.
To make Etel dust you must craft All Etel Gems (Basic Gems) first. To make the Beaker you need 25 empty bottles and 1 Earth Fourth Stone.
So you need to craft all the fourth stones in order to craft the Etel Gems and beaker, then you need Etel gems to craft Etel Dust. Then with your Etel Dust and Beaker you can craft and individual Master Gem.