"The Stone" is about exiles, of which Caine is one. He meets Montoya (Moses Gunn), a Brazilian son of a former slave and master of the Afro-Brazilian fighting art of capoeira. Montoya has a giant diamond that he is going to use to make his fortune, a quest he sees as deeply personal because it will lift his family's legacy from the shackles of slavery.
Caine also meets Zolly (Gregory Sierra), an Armenian who has fled the Russian (or Ottoman? It isn't clear) conquest of his country and wants to raise enough money to return as a freedom fighter. Only Zolly falls in love with a frontierwoman and her three kids, something that doesn't fit into his plans.
The convoluted plot (Montoya loses the stone to Zolly's potential kids while Caine gets involved and is being chased by bad guys and a marshall forcing Caine to confront Montoya and Zolly to confront the marshall and his purpose as a freedom fighter) doesn't bring these elements together efficiently, and in the end we're left with an episode that is spread too thin and relies on too many strange connections. So, honestly, I gotta give this one out of four yin-yangs.
Mark Pollard at Kung Fu Cinema has rightly pointed out that even having a capoeira theme to an episode is a remarkable example of the show's forward thinking. The art is well known today but in the 70s? Moses Gunn you'll recognize from a number of blaxploitation movies, including the Shaft films and other work. He's always seemed really wooden to me. Gregory Sierra you'll recognize as Julio from "Sanford and Son" and Sergeant Chano on "Barney Miller." Sierra gives a terrific performance here. Also of note - not all the fight sequences on the show were equally fun, as this episode shows. Lots of cutaways of Montoya's foot going into a bad guy's stomach, etc.
IMDB for this ep is here.