Got a pic of the leaves or a link to some place online so we can see what you mean?

I wonder if what you have seen is using a zylene based sealer. It's pretty stinky toxic stuff but does have a real depth to it which ends up making the leaves have more of a glazed pottery sort of look.
Cindy

I was 4 1/2 hours away in NC and they were at a few nurseries. A large rhubarb leaf for $400.00 and a smaller one for $300. A medium sized hosta leaf for $50. I didn't get any pictures unfortunately. It did look like a glaze, you could see thru the thick clear coat. They were very expensive.