Is there some unwritten rule somewhere about which type of resource islands the explorer finds? When I first tried this out the explorers seemed to bring back a nice variety of islands, and of course I started off picking titanium since I'm not a high level player and can't produce that on my own island. When titanium wasn't an option, I would pick gold ore, because it's always nice to have a little extra. But after a while, those islands seem to have become scarcer and scarcer, so I started to pick iron and granite. Now it seems the only resources my explorers ever find are marble, coal, and hardwood—which aren't too exciting because I can produce plenty of those on my own island.
Is there a rule somewhere that if you claim islands of a certain resource, then your explorers will no longer find that resource? I've looked around and can't find this explained anywhere. Does anyone have a link to where it is explained how this works? My main interest in doing this at all is upgrading the water castle, but getting a flow of resources that aren't easy to come by otherwise is a nice perk. I wish I had a better understanding of how this worked.