Riddle

In primary school my homework was usually just Hiccups and the occasional project... except for one day, in year 6, when our teacher (who was incredibly evil and hated me with an undying passion. She made me cry on my birthday. My BIRTHDAY. Evil) decided to give us a riddle to solve by the next morning. She said whoever could solve the riddle would get a treat of some kind (I'll mention here that my favourite teacher (who taught me in grade 3 or 4) got in a heap of trouble for giving us lollies, and yet this EVIL teacher gets away with it. What's with that?) and so, naturally, we all went home with candy on our minds and a riddle in our hands. The combination was all but fatal. The next morning there were tears, laughter, tears of laughter and a room full of devastated children (excluding, of course, the one girl who guessed it and spent the day tormenting the rest of us less riddle savvy kids.)

I stumbled across that same riddle just now. And hey, it's not like I've got anything better to blog about. So here it is, have a guess. I thought it might inspire some of you to comment. Possibly.

In marble walls as white as milk,

Lined with a 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 and steal the gold.

What is it?