Hi!
I'm just getting started in the hobby and reading lots of books and I hope you won't mind me asking a question based on your photos.
I really like the whole idea of decorating your fish tank and I've read that slate is an ideal rock to use as it is "stable". But I've also read that you should avoid rocks with "brownish" streaks in them (as there seem to be in your photos) because they could contain iron which we don't want in the water.
Are the books wrong or is slate an exception? Have to ask because I'm collecting every interesting rock I see, at the moment, that may be suitable for my tank when I set it up.