Jeannine aka The Lovely Sinner

First published as The Saint and the Lovely Sinner in Argosy For Men (Feb. 1948), then as The Saint and Jeannie in Pocket Book Storyteller Weekly (Jun. 4, 1949), as The Lovely Sinner in 3 issues of The Saint Detective Magazine (US - Aug. 1958, AUS - Aug. 1958 & UK 0 Oct. 1958) and Suspense (UK - Feb. 1960). Also published in two collections: Saint Errant and The Second Saint Omnibus.
A dictator owns a priceless pearl necklace that is the target of more than one thief.