materials advice

I am working on a project right now where the feeling needs to be that it is a hard outer shell protecting nature (yes it is school, and my masters thesis i would normally never design philosophy). I am trying to incorporate the feeling of outer protection that hides something very organic and natural in the middle . the middle is both nature and beauty … so the product on the outside will be stamped steel or aluminum. like a can. the inside, i am having problems with. i amost want to communicate that it comes from nature but am unsure of what material to use to show that… please help.