Firstly, Saruman made his own ring of power independently, using his extensive knowledge of Ring Lore to create it. By doing this he hoped to operate as an independent force in Middle-earth; it was Saruman's 'Third Way', as he hoped this would place him not on the side of Light, but also not on the side of Sauron.
The darkness which accompanied Sauron's forces into Gondor must have been produced by Mount Doom, and is possibly a good reason for him 'firing it up' again; I often wonder if that darkness was as a result of pyroclastic flow, as it sounds in some ways very similar to the volcanic effects seen at Pompeii and other famous volcanic eruptions.
Now there's another question in that did Sauron 'control' Mount Doom in any way? Or did he return only once it had begun to be active again? Or was his return the catalyst in its becoming active once more? Sorry, that was actually
three questions...
