Sherlock Season 4: 20 Easter Eggs & References From The Six Thatchers

11. Black Pearl Of The Borgias

No, this isn't a Pirates of the Caribbean reference, but - surprise surprise - a nod to the canon. In fact, it's another reference to The Six Napoleons.

In The Six Thatchers, Sherlock believes that the culprit is hunting for the missing Black Pearl of the Borgias, which he theorises has wound up inside one of the Thatcher busts. He's wrong on this occasion, of course, but the red herring is actually the solution of the original story.

Interestingly, Mycroft says that Moriarty was once interested in the pearl, which means it could be of bigger significance down the line.