May I point out that the movie has one humongous flaw in the
plot, in terms of period social
etiquette? Namely, 17-year-old girls could not properly accept as a gift from a
suitor large cut stones. She couldn't even wear them.
The primary reason for this was often given as aesthetic: extravagant jewelry connotes maturity and married life, to do otherwise would distract from a younger girls' dewy freshness. The real reasons were more utilitarian: it's ostentatiously reckless to allow into the hands of a relative child large sums of money.
Also, while many respectable women of the time wore extravagant amounts of jewels, the very largest stones were often given to the prostitutes known as "les grandes horizontales" by their clients as publicity for both of them. To give such a gift as an engagement present (even rings weren't all that common at the time, and they were, many times, not diamond, either) would have been considered a deep insult to her family. Only later did De Beers promote the idea of diamond engagement rings, the larger the better, as de rigeur, ironically enough by offering preferential rates to actors in Hollywood.
Of course, to point this out would have made the First Class passengers seem a little less mercantile...