Cool. I love rocks of all kinds, doing some flagstones and sitting boulders at my house soon, myself.
My agate post was inspired by finding the biggest one I've gotten yet:
http://farm3.static.flickr.com/2629/374 ... afe7_o.jpg
Weird thing was, I didn't even know it was an agate til after I'd picked it up ... was on mushrooms actually, by myself out in a river in my underwear for 5-6 hours (got a great sunburn), was thinking about the interplay between Flow and Form, when I saw a rock in the stream and for some reason heard myself say "that pretty much sums it up" and grabbed it - but didn't even realize for several minutes that I'd found a huge agate, until I'd cleaned some of the algae and other life off of it.
PS - how agates happen is an unsolved mystery:
http://www.agatesofscotland.co.uk/Agate%20Formation.htm