Per Ankh wrote: Not counting the 8.3-repeating percent of the planet's population born in May who have emerald as a birthstone.
You're right, I forgot about the birthstone thing too. Pearl is mine, but it's taken. Thought it was freed up during Steven the Sword Fighter, but that wound up being a false alarm.
I don't recall anyone saying two gems can't have the same stone, do you? So long as it isn't a carbon copy of Pearl herself, I doubt anyone would actually have a major problem!