Finding a diamond in another country

Let’s say you are travelling to Botswana and happen across the worlds second largest diamond. You get it through…unscrupulous means and plan to bring it back to the US.

As long as you declare it at customs in the US, can they book you with any crime? You probably had to hide it while taking it out of Botswana and with a diamond that big there was probably some shady dealings, but if you declare it at customs can any of those crimes follow you?