Marble walls as white as milk, lined with skin as soft as silk, in a fountain crystal clear, a golden apple will appear, there is no key to this stronghold, yet thieves break in and steal the gold. What is it?

Common Wrong Answers
“A coconut”
While a coconut has a hard outer shell and can contain liquid, it does not fit the description of marble walls or the soft skin. Additionally, it does not have a golden apple inside.
“A pearl”
A pearl is formed inside an oyster and is often white and shiny, but it does not have walls, skin, or a fountain. It also lacks the imagery of a golden apple.
“A marble statue”
A marble statue is made of solid marble and does not contain anything inside, nor does it have the soft skin or a golden apple, making it inconsistent with the riddle's clues.
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