Deceptive Diamonds

While people say that “all those glitters are not gold”, similarly “not all stones are diamond”. What was that thing which made Ariel regret her decision of showing off her ring? Was her diamond ring an artificial one? Or was it too expensive to be real? Maybe her ring was a lot different from people had anticipated it. Whatever it was, Ariel had to pay a high price for it. Yet, again, why?