I honestly agree with the savings account advice. Otherwise, maybe a pearl. In my country (somewhere in Europe) some jewellery stores sell individual pearls along with a box to store them. When you have enough they can make something for you with them. You might have to get several a year to get enough for a necklace at 18, but a standard cultivated pearl should be below 75$ so it should work. You’ll have to ask around if it’s a thing at all where you live however.