My guess it's poured fondant - but......somehow made thick so it can be rolled out. Just a wild guess. Have no idea how to make such a rolled/poured fondant!
Could it possibly have clear gelatin poured over it? Not even sure if that is doable but when I blow the pic up it looks as though it has a clear coating of something.
That's not a cake, it's glass.
The flowers and side swirl could be done in pulled sugar. I wonder if that top 'cake' could be done in blown sugar.
http://www.sweetandshiny.com/art-ivory-cake.htm Neat though!
That's funny. (Good catch, AZ)
You can also get a shiny effect on very smooth fondant by brushing it with melted shortening.
you wont be able to get a cake looking like that glass object, all you can do is say to the customer that you will use that as inspiration, but it will not be exactly the same.