Playtesting is going well, and I'm pretty satisfied with everything. I also have cave coral done, and there's a poll now regarding them. I'm using the unusual egg mechanics to have them give stones, and it's a bit of a pain to set up proportions of various stones, so I'd really rather only do that once, but I'm not sure which setup will be best.
I'm currently playing with an incomplete regional setup - I made cave coral for metamorphic, igneous intrusive, igneous extrusive, and sedimentary stones, and each one gives an approximately proportional sampling of a subset of the stones available in those regions - mostly layer stones, but with a decent chance of large cluster stones and a smaller chance of vein or small cluster stones, and no gems. So far, that's been pretty neat, though I wouldn't want to rely on it for any industries, even with the 20%-per-coral-per-season chance of getting flux stone out of the sedimentary ones. If I go with that, we'll need some more reliable way of getting ore and other useful things, probably either with specific-stone-giving coral or coral that are based on the various large-cluster stones and give a higher proportion of the stones that would be found within those clusters. (Say, a 20-40% chance of getting a stone that's usually found in veins or small clusters or singles, and a 60-80% chance of getting the base large-cluster stone.) (Yes, this does mean that magnetite coral would exist, though they'd be pretty insanely expensive.)
Another option would be to have a coral for each layer stone, with a somewhat higher chance of getting non-layer stones but a smaller selection in each one compared to regional coral. I could also go with my original idea of making one coral species per stone type, but there are a lot of stone types, and that might clutter up the buyable-animals list a bit too much. (Taking out the existing aboveground ones will help, though.) This may also make it a little too easy to satisfy that rutile-loving mayor, though with the current state of the caravans I'm not sure that's a bad thing.
Left to my own devices, I'll probably go with the region-plus-specific-stone version, but I'd like some feedback on this one.