I am not a geologist in any way. I gave you what information I could find. The only answers I could find said they were caused by impurities. The rock hounds here had a chance to answer your question. I am a guy who responds to the questions like any other poster accept I can approve comments and questions. I was on a ten day trip to Lake Superior, collecting agates and the like. Next summer I hope tp sift through some slag from the copper mines in the Keeweenaw Peninsula. During that time, if the moderators do not approve things, the posters will wait until someone shows up, likely me when I get back. I often spend my own time searching for answers for posters here. I am sorry I did not meet your needs in a timely fashion. I was catching my fantasy football teams up and did not get back to further research the question.