In She Stoops to Conquer the theme of "the marriage by convenience" permeates the plot. It demonstrates the dependence of women on "marrying well", which basically means that marriage was used in the 18th century to solidify networks, to expand wealth, or to preserve hard-earned riches.
The novel has no problem speaking about the "marriage market" and how parents carefully plan their sons' and daughters' engagements by selecting from a list of well-to-do, or landed heirs.
In the case of Miss Hardcastle, she is lucky because the man that she is supposed to marry, an heir named Marlow, is actually a good man. She falls in love with him after their first chance encounter and she is able to see the good in him. However, it is clear that money does play a role in determining social acquaintances.
As we know, Marlow is attracted to lower-class women and this poses a problem for him; men of his class are not supposed to mingle. This is why, when he mistakenly confuses Miss Hardcastle with a barmaid, he does explain to her that, aside from him being promised to someone else, he also knows that the social classes are not to mix:
Excuse me, my lovely girl, you are the only part of the family I leave with reluctance. But to be plain with you, the difference of our birth, fortune and education, make an honourable connexion impossible.
Therefore, money is a denominator of class, which is a denominator of lifestyle.
Meanwhile, the jewels are also symbols of being marriageable. Mrs. Hardcastle and Constance both inherit colonial jewels that, as a dowry, would set them as worthy of getting a good husbands. The jewels are so valuable that Mrs. Hardcastle even keeps Constance's share of them under her own guard. It is because of the ownership of the jewels that Constance is able to elope with the man that she actually loves and the key to her freedom from marrying Tony. Therefore, the jewels, like money, signify the monetary value that makes someone of this place in history "worthy" of a good marriage.