The treasure of life
As the title suggests, this novel has a central theme involving priority and value. The idea is basically that by having one's priorities in order, one naturally discovers what is valuable in life. This theme is symbolized in the prelude to the novel, a short story about family resolving a disgraceful feud in the desert. As they die, they discover that the love of Nell's parents was authentic and sanctioned. They also find gold. This points to the theme of value; it is valuable that Nell's honor be redeemed by love. That process is shown as the main treasure of life.
Honor and disgrace
For some readers, it will be important to clarify that this novel's ideas about gender and sexuality are clearly outdated, but still the traditional roles can be interpreted by abstraction. For instance, it is so important to Nell that she not stain the Gale family name if she were indeed found to be illegitimate. Now, that is a painfully outdated idea, but it is symbolic and thematic: It shows to Gale that she has high honor. Her compellingly honorable behavior is a highly prized aspect of her character, and her loyalty to goodness (even in her traditional understanding of it is proof to Gale that she is ideal.
Redemption and romance
The undertone of the novel is that through the process of falling in love, one learns to celebrate a new person the way they are. That is a redemptive process, because by loving and accepting someone, forgiveness is exchanged. This forgiveness takes a symbolic form in the novel, but the symbolism is clear: By accepting Nell even if she is not "socially acceptable," Gale is rewarded for his socially optimistic point of view, and Nell's honor is restored in the process. They also "find the treasure of her dead forefathers," which is clearly a symbol referring to romance as an inclusion in one's family and inheritance. Now they share in her inheritance, symbolized by the man's promise to give "half to Nell, and half to whoever else finds it."
