Let's be pragmatic here. Prior to, say, the 1920s unmarried women had to be very cautious of their perceived "virtue." Most women were unable to support themselves in anything resembling a comfortable lifestyle because they simply weren't hired for jobs outside of factories and elementary classrooms, so they had to buckle down and keep themselves marriageable. Giving an expensive gift other than an engagement ring was a way of saying "hey, I'm not going to marry you, but I'll do my bit to support you financially because I'm grateful we're doin' it." The song "Diamonds Are a Girls Best Friend" is a slightly later manifestation of that understanding. Don't accept jewelry lest everyone will think you an almost-prostitute was kind of the conventional advice.
I have received very expensive gifts from men I was dating. If the gift was gotten because the giver knew it would please me particularly and he could easily afford it, I was happy to accept it. If the gift was meant to show off or was given with expectations of sex, I was equally happy to refuse it. If you don't know which is happening, ask. If you're uncomfortable with the answers or in general, graciously refuse.