A heart pendant necklace in sterling silver set with two birthstones, with every combination of months offered, supplied in a gift box.
A double birthstone says something a single one cannot. A mother and a child, a couple, two siblings: the piece carries both dates, which is why it is the one people buy for an anniversary rather than a birthday.
Pick the order of the stones deliberately if the months differ in colour. A dark stone next to a pale one reads better with the pale one outward.
Sterling silver rather than plated, because this is a keeping piece. Silver can be polished back after years; plating cannot.
It arrives in a gift box, which matters for something handed over on a date.
Polish it with a cloth and keep it out of a bathroom, where damp dulls silver fastest.