Gold bar

 A Gold bar is a bar of refined gold created through the Smithing skill at level 40 by using a gold ore with a furnace, or by casting the Superheat item Magic spell on a gold ore.

It is frequently used for Crafting and can be bought from Bob for 300gp.

Bob can be found through Spellbook > Skills Teleport > South Neitiznot > Left of the Furnace