Can't find Jet or Plumbite

Look around your lot after a thunder storm. The lightening strikes leave crystals, and other stuff. I know I got Jet that way. I have one house with a lot of flat roof space. I've found them on these roofs and around swimming pools, especially after storms.

I don't have a list of where i got them but rare ones are more likely to be in the secret areas like forgotten grotto and the glade. Maybe the secret areas need to stay loaded to respawn there so you could try camping there to pass time if you have tents

I think I got jet in Selvadora. I also got it from a lightning hit.

Having completed most of the collections in the game, I've concluded that the last item you need, whether 'common', 'uncommon' or 'rare' will be impossible to find. You will spend days looking for it. I finally got a diamond by accompanying my scientist Sim to work (there are lots of spawners in the secret science area). Later, I visited Selvadora and got diamonds by the truckful, only because I no longer wanted one.