Yes, stone. Everything in the cities looks like it's made of stone, but it's actually made of ore. Iron – isn't that ore? But fabric and clay with sand aren't needed; that's just unnecessary complexity. Gold, of course. But we need something else for hiring high-level creatures! And that's where crystals (or gemstones) fit in perfectly. And food is unnecessary. The warriors will provide for themselves; we'll be giving them gold and crystals, won't we?