The jewel refers to Sidi, a young girl who is trying to decide which man she wants to marry.
Sidi is called the "jewel" because she is beautiful and coveted. Baroka and Lakunle both want to marry her; she is able to choose between them. She was in a magazine, and Baroka flatters her by telling her how beautiful she looks. Even though she isn't inclined to marry him when she finds out that the much older man wants to marry her, she ultimately does when he tricks her. She believes he is impotent, so she sneaks into a feast he's having. By the time she leaves, she's planning a wedding.
The jewel of Ilujinle ultimately marries the old-fashioned man instead of the modern one. Baroka promises to put her face on the village stamps; she tells Lakunle that Baroka has more life in him, despite his advanced years. She also finds out that Baroka isn't as stuck in his ways as she originally thought. For example, he lets his servants create a union.