You want to build a curved staircase out of wood, but you dont want to bend it. This does not relate.In any case placing the treads with deck boards would be the easy part.

Marine plywood is the same as any other plywood far as exposure to the weather. The Marine part define is there are no knots in the ply,clear plys, its a strusture thing not a weather proof thing. J.

you might consider looking into a tile forum like john bridges (google it). Look into using using a product like ditra on top of plywood/CBU followed by a decorative stone

Here is a link that might be useful: DItra installation

I'm sorry, I am no longer using deck boards. My finished stairs will be stone. I need help in building the round substructure(out of wood), quarter round(90 degrees). Thanks for any ideas.

I just figured out, and cut, 2 correct stringers. I will need about 6, but I have not yet figured out how to attach and support them at the top. I will be attaching the 2 ends to the house but I don't know how to support the 4 center stringers at the top. ??

Maybe I don't get what you're doing...building a wood frame and putting stone on top of it??? That doesn't sound like a formula for longevity given that wood will rot long before stone crumbles. Not to mention you'd need to engineer it for a pretty heavy dead load, not the 10 psf you do for a wood deck.

Generally stone is laid on a base of dirt, sand, gravel, etc. (i.e., other stone-like material of different diameter). Why not just build out the existing stairs with more concrete or cinder block, then face them with stone?