I'm really puzzled now! Not only do 'Stone' and 'Flint' consist of five letters each when the question demands a 10-letter answer, but I can't see what they have to do with the material in the puzzle. Is it me?
Thank you very much bridgnut and teacher1, think I will go for Flint as it is possibly more of a mineral, had seen Stone before but wasn't sure. Would like to give you star ratings, but for some reason I haven't had the option over the weekend, it only has report this answer! any ideas why? Thanks again.