The Roman Mysteries: The Beggar of Volubilis: Book 14 by Caroline Lawrence
Flavia and her friends are on a quest for the Emperor Titus - to steal a valuable gemstone known as 'Nero's Eye'. The Delphic Oracle prophesied that whoever owns the gem will rule Rome - so Titus is determined to claim it for himself. Their travels take them across the Roman province of Mauretania, from Sabratha (in modern Libya) to Volubilis (Morocco). As they travel on a caravan across the desert they encounter slave-traders, pantomime actors and a wild animal stampede. The detectives must consider another quest: what has happened to Uncle Gaius? Meanwhile, Flavia faces some tough decisions about her future.