Mary of Burgundy
While diamonds and engagement rings both played separate parts in different traditions and cultures, it was not until 1477 that both paths intertwined.
After deliberating which suitor she'd pick as a husband, in 1477 Mary of Burgundy accepted the Archduke Maximillian of Austria's proposal. He proposed with the first ever diamond engagement ring on record, which he had commissioned. This started a trend amongst nobility and aristocrats to follow suit.
More than half of a millennium later, this tradition still remains strong.