I don't know the answer to your question, but I have passed similar stones and wondered exactly the same. I also speculated that such coated stones may come from the gallbladder, not liver. There are usually only a few such stones and they are always dark blue-green inside, while other stones have been any shade of green.
I think they might be from the gallbladder because on my 3rd flush I passed 5 or 6 big stones, over 2 cm long, that looked just as you described - hard tan shell and blue-green inside. Since that flush I have not had any more gallbladder symptoms at all. Other such stones in later flushes have all been small, pea-size.