OK... at the risk of being the lone dissenter and putting a target on my back in this post...
First, with apologies to Halftail and Roundball, both of whom I have a great deal of respect for, flint is porus in a sense - the crystal lattice can, and does, take water into the open space.
Second, the properties of flint can be altered, through either heat treatment or waterlogging; for a brief, non-technical discussion check out
[url]
http://www.geocities.com/undyrm/treatment.html[/url]
Freshly dug flint does tend to have a higher amount of water in the lattice due to being isolated from the atmosphere to some extent; the covering material provides a more stable environment. Upon exposure to air the flint begins to lose water until it reaches equilibrium with the air. Some flint knappers I've met prefer freshly unearthed material to work with, or keep the flint stored in water for a while until ready to work with it.
The real question is: does this effect us to any appreciable extent in firing flintlocks? Are these treatments worth the time and trouble? I began storing some of my flints underwater about 20 years ago after hearing about this, just to see how they compared to flints stored in the dry. Over a couple of years I noticed that the "wet" flints generally lasted longer than the "dry" flints, and suffered fewer major failures (such as shattering). I didn't notice any real difference in the sparking. Now I store all of my flints in a plastic, water-filled jar, and order ahead so that all new flints have at least 3 months in the water (different jar) before I use them. No, this was not a rigidly controlled scientific test, but I noticed enough difference to go ahead and store all my flints in water (gotta store them somewhere anyway; this way they all stay together and don't migrate to random corners of the house.) I've had some anecdotal, third party reinforcement for this policy when friends to whom I loaned flints (OK, you don't really "loan" a flint, you give it away...) ask me where I was getting the high quality flints from... Tom Fuller flints, just like they were using.
Is it worth the trouble to store them in water? I decided that it was for me; others may have different experiences. If you're not sure, try it and find out one way or the other... at worst you'll have few wet flints with no change (they don't go bad in the water!); maybe you'll have some change that isn't worth the trouble; at best you'll have a new trick for your rockcrusher.
Best regards,
m