Tubes form in some of these agates. Agates are thought to be formed in volcanic rock which when molten has air pockets. Later, silicate rich waters seep in through these holes, evaporating and becoming a layer of mineral. The different colors come from different minerals. They were formed 1-2 billion years ago. They may be the original seams by which the moisture seeped into the agate. The agate lady has a web site and a book on agate formation which might be helpful as it goes into greater detail.