Geekdafied--You are correct, what you have is probably limestone and good for your aquarium as far as buffering is concerned. However, it probably has a ton of phosphate in it. TN and AL limestone is packed with phosphate. If your brother is a geologist, he should be able to tell you this. The rock also probably does have some amount of iron and other non-reef safe minerals. There was a huge article written on base rock 5-7 years ago where a geologist researched rock makeup all over the country. He/She showed data showing that if you want calcerous rock without phosphate and iron, you need to get it from the Miami/Keys area. I would not put what you have in my reef. Your welcome to do what you want with your tank, I just wanted to pitch in my two cents.