Here there is no available stone in the forge. As stone is smelted from the input slots, the available stone in the forge will increase.
Stone is a material used by the Forge for crafting items from forge recipes.
Once a source of stone such as Small Stones has been smelted in the forge, it becomes available as stone. The stone for a recipe must already be smelted in the forge before you can craft the recipe.
For a complete understanding on how to smelt stone and other materials, see Smelting.
The following recipes require Stone (Forge): (NOTE: Times are based on a level 1 skill and the material that an object is crafted from, which is only reliable for food at the moment, so is rather difficult to calculate, this table is auto-generated):