Reply #149: If you ever really NEED to know how to do this, you're in serious trouble!

[Richard Steele]

Beat some the dry wood into as fine a pulp as you can with the rock,
until you have a small pile of very fine fibers & shavings to start
your fire with.

Find the CLEAREST section of ice you can- you'll need a piece 5 or 6
inches thick, big enough to carve into a 18-24 inch circle.

Then, by rubbing with the rock, grind that piece of ice into a bigass
MAGNIFYING LENS. When the shape is right, friction-polish it by rubbing
briskly with your gloved hand. (assuming your gloves aren't back at the
house with your matches)

Obviously, ice doesn't make as good a lens as glass does- that's why you
need such a big one. But an ice lens that size CAN focus enough sunlight
to ignite that bed of dry wood pulp you prepared.