I had to google "peacock stone" to try and figure out what this stuff was. Michael's crafts isn't so good at labeling with the chemical formula. I should consider myself lucky for getting "peacock stone". One questionable internet source suggested it was also known as bomite, but that looks like an alias for chalcopyrite, which this is not. (though chalcopyrite is sometimes referred to as "peacock ore.") The circular bodies do look a bit like oncolites, but I don't get a particularly sedimentary vibe from these guys, and they would have had to have been replaced by silica. Unfortunately, I'm still not sure. I can't find my mineralogy textbook anywhere either.