It's tradition to gift a "royal" bride with something from the grooms mother. William got the sapphire, Phillip made Elisabeth's engagement ring out of stones from his Mothers tiara.
The way I heard it (if it’s true) they were allowed to choose from their mother’s jewels, which she wanted them to have for their future wives. Harry originally chose the sapphire, but then when William became engaged, he gave it to William for Kate.