My nephew found this rock in a stream in Colorado and thinks it may be an egg fossil. It appears to be 2 very different types of rock, with the inner one being very smooth and rounded. Thanks

It looks like a geode to me.

geode - Google Search (http://www.google.com/search?hl=en&q=geode&bav=on.2,or.r_gc.r_pw.r_qf.,cf.osb&biw=1024&bih=601&um=1&ie=UTF-8&tbm=isch&source=og&sa=N&tab=wi&ei=rRv-T9_LC4bCrQGSscSLCQ)