The king's daughter Lady (Daisy) is with child by a kitchen boy. The king has the boy killed and a token (his heart) sent to Lady Daisy. She dies for love (prompting the king's deep regret)
[A. L. Lloyd writes,] "Boccaccio re-tells [this story] in his tale of Ghismonda and Guiscardo, and in later years it was made into a play in England and elsewhere. Versified into a ballad, it was widely known throughout Western Europe and Scandinavia." - PJS
The link to Boccaccio was noted long before Lloyd; Child mentions it and many non-English analogies, and the link to the Decameron goes back at least to Dixon.
The tale is the first story of the fourth day, told by Fiammetta. In outline, the Decameron account is precisely "Lady Diamond," but there are also substantial differences. In "Lady Diamond," the girl is pregnant and the father forces the truth out of her; in Boccaccio, she is already a widow and her father discovers the truth accidentally; in "Lady Diamond," she dies for love, whereas in the Decameron, she takes poison, and the Italian tale ends with the king's repentance, something rare in the ballad.
With all that said, it's hard to doubt that the two spring from the same sources. Much of the difference may be simply due to the fact that the Decameron version had to be fleshed out to a full story, while the ballad version, like most ballads, strips much inessential detail. - RBW