1477 CE
An archduke seals a betrothal with a diamond.
Read the full sourced page: The First Diamond Engagement Ring
Myth: undefined
Reality: Betrothal rings are ancient, but a Roman bride wore gold and iron. The first well-documented diamond engagement ring is from 1477, and before the Second World War only about 10% of American engagement rings had a diamond.
The Gem Tray is another reference site by the same author.