Didn't say you had to like the rule! I'm just the messenger.
For instance, from Miss Manners (who doesn't live in 1912):
All presents are laden with symbolism, Miss Manners warns you, but jewelry is explosive with it, and never more so than when given by a gentleman to a lady or relative to one of the next generation.
Ladies should never confuse gentlemen by accepting jewelry if not prepared to accept the gentleman who offers itónor by criticizing a proffered ring when intending to take the gentleman himself.
Is that "accept for marriage," though? I would say that gifts of jewelry are
meaningful, in general, but that the meaning is not always marriage. Certainly, that's the meaning of an engagement ring, and one shouldn't accept an engagement ring unless one accepts the engagement. But I think other pieces of jewelry can have other, intermediate meanings.
For example, I know that in some times and places it has been a big deal to get the guy's fraternity pin (don't know if people still do this), but it didn't mean you were engaged, it was more like going steady. I would say, in general, that if you accept jewelry of value from a guy, you're conveying the message that you accept him for a relationship
, but not necessarily for marriage, depending on the context and the type of jewelry.
(And of course this is also subject to common sense; none of us are required to marry a boy who gave us a jelly bracelet in grade school.)
ETA: And I looked up the column, and the context from which she's jumping off is actually a piece of jewelry given by a father to a daughter, and her main point is that the daughter shouldn't criticize the gift to the father's face unless she's prepared to face negative relationship
consequences. The romantic aspect is kind of a tangent she went off on and didn't go into great detail about.