A riddle

[hiflier]

There is a riddle in the form of a poem. The answer is 'an egg'. All that I can recall is a line containing the words 'halls of marble'. Any elucidation would be welcome.

[JackDanielsU]

Here ya go

In a marble hall white as milk
Lined with skin as soft as silk
Within a fountain crystal-clear
A golden apple doth appear.
No doors there are to this stronghold,
Yet thieves break in to steal its gold.