Greyhawk were touched by Rarity's generosity that no other ponies have ever displayed to him before coming to Ponyville. In return For Rarity's generosity, he picks a hooful rock and casts a spell with all his might and stamina to turn it into a diamond that plays music (Despite his lack of compatibility with the spell itself) to give to Rarity.

I like to thank Asbel Lhant for putting up that ask rarity thread to help me come up with that romantic scene and I say I did enjoy rping that scene a lot.

Rarity © My little pony friendship is magic, Hasbro
Greyhawk and art © Me