My Tresure

I have marble walls of white,
So fine, so soft.
Hidden treasure burried inside,
That can never be discovered
until it breaks out.
Nobody can seal my tresure,
no thief so experianced
as to break inside.
My marble walls are my locks,
There is no way in,
just a way out,
yet a few weeks,
and where is my tresure?
Gone, disappeared.
Or has it?
Has it broken free at last?


What am I?
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An egg!