A few sets made with very basic tools - shards of flint and slab of slate. Our stone age ancestors were very skilled with stone and could make far better stone tools than these so they would have been more than capable of making friction fire sets with stone tools. As I have found flint is very sharp and even with small pieces of flint like the ones below , it is possible to make friction sets - it just takes a little while longer. The slate (photo 1 ) was very good for processing the hearth board by scraping the wood over the sharp edge and by sawing the wood up and down the edge it made an excellent v notch (better than some I make with a carbon steel knife!) 

This is an interesting method :) A great work out for your legs :)
Again just a bit of fun, but it works well once you get the co-ordination right !

This one was just for fun - using my arm as the bow ! I don't take credit for first trying this, qute a few people have tried it and @wur_thaz_smoke had previously cracked it. It's not as easy as it looks here and takes a bit of experimentation to get it right and to get the co-ordination :) A longer spindle helps - this came just below my knee; and it was quite thin so less pressure needed; multiple wraps of the cord were also needed and I found it easier if the end of cord in my hand came off the top of the wraps rather than the bottom. 
I can't think of any situation where I would need to use this technique - as there is usually a bow of some sort available!