I understand that from a balance perspective metal was added to make stone structures difficult to make, but from a realism perspective - it's not realistic at all. Now i understand the game isn't a 1:1 copy of what was actually available to pirates but at the same time there is some forms of realism in there and they are important. So i suggest the following:
Replace metal with the need to use stone bricks. Stone bricks would be made from stone in a new crafting table called "Hammer and Chisel" and they would take a while to make. Then with the stone bricks, regular stone, fibre and thatch (and if you really must add it, wood) the stone structures are made in the smithy.
