Hi Erin: To my knowledge, the only place in the U.S. where you can mine gem quality diamonds is at Diamond Crater Park in Murphreesboro, Arkansas. It was, for years, identified as the only diamond mine on the North American continent, although that may no longer be true. It is unique in that it is not a commercial mining operation but rather a tourist attraction where one can pay a fee and dig, keeping whatever is unearthed. People find sizable stones all the time. I think most recently someone came up with a 4+ carat beauty, but there have been stones as large as 16 carats or more. If you google for Arkansas diamond mine you can find all the info you need.
As for Lex's enterprises, we know he was in real estate destruction so that he could re-devlop the area (the toasters in IGACOY). He wasn't involved with STAR labs but had his owns scientific research going (Vatman). We also know he was into R&D for defense projects (HIM) and he was developing a nuclear power plant (I think that was in MOSB.) Lois worked for his TV news station, LNN, after the Planet was destroyed. He owned the Lexor Hotel, and, of course, owned the Daily Planet for a while.Also in one of the eps in the troll arc, L&C look at a list of corporations and Lois categorizes them, remarking that those were things that Lex had owned.
It's interesting you should ask that particular question because I'm just finishing a story in which I refer to the many corporations he owns. I just made up what seemed approriate in addition to what I knew.
Hope this helps.
Jude
