I've made a bunch of these, am putting one up on ebay next week. I media blast mine with 220 grit aluminum oxide. The tend to squeek and the pitch is pretty high so drop your soundboard a little lower than usual. I've had customers tell me that the conditioning stones work over top of my media blasting but it literally takes hours of conditioning to get some good, quality grooves.
For anyone that wants to make these, go on ebay and buy the cheapest coasters that you can find that has this agate in them. The agate itself is basically lava.
You wont be able to shape it with a belt sander. I have had success cutting it with a side angle grinder, it's much like cutting steel, lots of heat and sparks. It's best just to buy the agate and work your call around it's shape. Not much of a difference between using different soundboards. The agate you are going to find is going to be much thicker than the standard .060 thick striking surfaces that you normally use.
All in all agate pots are mostly for show.
These pictures, three of these "agate slices" were cut from coasters. The blue one I cut by hand. The black one is going into a Gabon Ebony pot. The cream and light blue one shows how i sign mine with a power carver.
Jack "Pappy" Lewis is another experienced guy that works with agate.