Is it improper for a man to give jewelry as a birthday gift to a married woman?

Absolutely improper. Unless that particular married woman was secretly dating the man that wants to give her jewelry. To not decline the gift would be leading the gift-giver to think the married woman is interested in him. If that is the case, then she should go for it, otherwise a firm 'no thank you' is called for. As a married man my wife would be p... I mean she would be mad. Besides if I gave jewelery to another women she woul mean more to me than just a friend. Unless she was family or something like that. I would say in my case that it would be innapropriate.