Jewelry that is passed from person to person takes on a life of its own. Wearing a family heirloom can make you feel more connected to those that came before you, and passing it on connects you to those yet to come.
There are no actual rules for what qualifies as an heirloom; the emotional value of an item cannot always be measured. There is a common thought that there has to be a specific monetary value associated with a piece to be worthy of passing on. Pieces that are meaningful to you and the giver or recipient are more likely to remain in the rotation, as it were.
Any piece of jewelry can be passed on, but I would venture to say that the most common heirloom jewelry is associated with important milestones, especially weddings. If you are looking for something that will be a meaningful legacy, there are many avenues to travel.