I would guess silk, for the whole dress - and guess it's been gradient-dyed while bunched up or something like that.
You know how they have those "crinkly" dresses ? Kind of like that. You twist the wet silk and let it dry that way, and it will make semi-permanent pleats. By dying it while twisted up, I think you might get a look like that (like tie-dye). And you could steam it to make the pleats less stiff and more natural and distressed looking.
Disclaimer : This post written by a Grumpy Old Man
Actually in the pipes:
- Remaking Ser Loras doublet.
- Luke Skywalker, A New Hope
- Wind Waker starter costume in swim material
Eventually: Paul from Dune, Faramir, Harry Sullivan