Yea, I've done the sparker/block too, with some results. We're re-discovering 500 year old technology here so don't get discouraged. Also, try burying a dish rag at midnight.......I'm sure there's some Black Magic at work! Look at ebay for 'dollar pyrite', there were a bunch on there the other day, mostly from Illinois. I expect the best way to cut one up is with a tile wet saw. Though, I've scored with a dremel cutoff wheel and broke it. I guess there are several forces at work, wheel hardness and the specific reaction with the flint, pyrite etc., dog spring tension, Mainspring force. I re-read one of Razpla's threads here and he uses truck spring steel. The heaviest I have is from Dixie Gun Works. For example, mine won't kick open the pan cover. I'm thinking from what you say, that dog spring is a good start point! If you have some steel, might be better to make a spare first and try before messing up your neat filing!