Fixed tranquilite from being used in wall construction.

code/game/objects/structures/girders.dm

@@ -313,6 +313,9 @@
 
 		if(S.sheettype)
 			var/M = S.sheettype
+			if(M == "tranquillite")	//tranquilite walls don't exist in the code (yet)
+				to_chat(user, "<span class='warning'>You don't think that is good material for a wall!</span>")
+				return
 			if(state == GIRDER_DISPLACED)
 				if(S.get_amount() < 2)
 					to_chat(user, "<span class='warning'>You need at least two sheets to create a false wall!</span>")