Tarzan is roaming the jungle when he comes upon a half-dead American wandering alone. He is Stanley Wood and relates a strange tale to Tarzan. He has just escaped captivity from a tribe of warrior women. The women are divided into two tribes the Zuli and Kaji. Both are controlled by twin brothers who can control people’s minds with a large gem stone, a diamond and emerald respectively. The woman are obsessed with becoming white so they capture safaris with white men so they can breed with them.
The power of the emerald is too great for Stanley to resist and escapes Tarzan to return to captivity. Tarzan follows and finds out that he is immune to the power of the gemstones. He defeats the wizards and frees Stanley and his love Gonfala the Queen of the Kaji.
Later in the second part Stanley and Gonfala wander into the land of Athne the city of ivory from Tarzan and the City of Gold. Now Tarzan gets involved in overthrowing the corrupt rulers of both Athne and Cathne.
The twenty-first book in the Tarzan series is actually two separate short stories that are combined with one. I found it interesting with a slightly different twist in the lost civilization. He also revisits the Land of Othnat which is the only time except for Opar that Tarzan would revisit a lost civilization.