Peanut shells and potato skins are also on that list. Nature provides a protective coating or a layer of cells on tubers that are extremely difficult to compost. This is how these things can be underground and not rot. Same thing happens in your compost bin.

I've had the same experience with melons. The paper-thin skin remains.

Shaul

Also avacado skin looks like it did when you bought it!

Ok, thanks for the replies. I had not noticed about the potato skins and avocados, probably because I don't feed them to the worms, only in the compost pile.

Everything disappears if you wait long enough. Be patient. Avocado skins cycle through my composter 2 or 3 times before they're digested.