@DeepKling @starwall @adingbatponder Yeah. I'm sure the gray ones would form in neutron star mergers, but they're too unstable for them to still be around on Earth after all that time -- and they don't show up in decay chains of other things, etc. So, all of our *samples* of them were man-made, but that doesn't mean they don't occur in nature. They just don't occur on Earth naturally.