If you think agates look great inside a stripping guide, just think how nice a agate reel seat cap would look on the fly rod!
I built a fly rod with a French Style reel seat (two rings to hold the reel to the reel seat). I commissioned a Lapidarian (Stonecutter) to create a reel seat cap, using Lake Superior Agate (red-brown hue with bands of milky white veins. I had to let him have the reel seat for matching the stone to reel seat cavity. But I now have a lovely reel seat on a lovely 3 weight fly rod.
I would not recommend a French Style Reel Seat for any fly rod over 5 weight.