Episode 26 of The Trigan Empire
The City of Jewels
aka The City of the Jewels
Het geheim van de eeuwige jeugd (Dutch title)
The Secret of Eternal Youth (Dutch title, translated)
The City of Jewels was originally published in 8 installments in issues 481 (3 April 1971) to 488 (22 May 1971) of Look and Learn magazine.
It was reprinted in the Vulcan Annual 1977.
It is reprinted in the Don Lawrence Collection Volume 7, The Rallu Invasion.