The jewels in the play refers to the box of jewels that is constance Neville's inheritance left in the care of Mrs. Hardcastle, her aunt.
Mrs. Hardcastle does not want to release the jewels to the rightful owner, Constance. She intends to keep such inheritance in her family. Therefore, she covers up this intention by forcing Constance to marry Tony, her mischievous son.
Constance is wise enough to play along with Mrs. Hardcastle by letting her "suppose that I am in love with her son and she never once dreams that my affections are fixed upon another". Hastings is the one Constance loves and not Tony. Tony, on his part, prefers Bet Bouncer to Constance. Because he is aware of his mother's plan to rob Constance of her inheritance, he agrees to be in love with his cousin, Constance, in the presence of his materialistic mother.
Constance Neville is in no way ready to part with her inheritance as she confides in Hastings, her lover. So she pleads with Hastings to bear with her until the moment the jewels are put into her hands; then, their marriage can take place. The more Constance persuades her aunt to allow her access to the jewels, even if for a day, the more Mrs. Hardcastle insists on her marrying Tony. Meanwhile, Hastings discovers that truly Tony prefers Bet Bouncer to Constance. He, therefore, persuades Tony to help steal the jewels from his mother.
Mrs. Hardcastle tells Tony to bear false witness before Constance, that the jewels are missing. Ironically, the jewels are indeed missing because Tony has given them to Hastings to enable him elope with Constance. Hastings immediately gives the jewels to Young Marlow for safe-keeping who, in turn, gives them to Mrs. Hardcastle for safe-keeping too.
Tony inform his mother that jewels are stolen. The mother then goes to check and discovers that the jewels are really missing. She raises an alarm that she's robbed but Marlow tells her that the jewels are with him and gives her the jewels.
The jewels help to accentuate the irony, comedy and indeed the mistakes of the night in advancing the plot. Through the jewels, Hasting and Constance's plan of elopement is exposed. The jewels bring out the selfish and materialistic character of Mrs. Hardcastle.