Thank you for some very interesting suggestions, love the corten steel idea. I had to spend a bit of time on google to find out about some of the more exotic suggestions though.
Again, it comes down to availability (and cost!) in a suitable size billet and machineability.
Damscus steel is very interesting and easily available. groundflatstock.com have a beautiful and bewildering array of damascus. I'm not sure about machining it though – surely every batch is different and therefore unpredictable?
As this is a pocket watch and not touching any ones skin, wouldn't a material like aluminium be acceptable?
Steve