Onyx engagement rings? Absolutely! Diamonds may be a girl's best friend, but Onyx engagement rings are a unique, original, and a far more affordable choice. Black and gold make a beautiful and elegant combination. A black onyx ring is a perfect accessory for any outfit. Sapphire is the birthstone of September, and one of the most popular gemstones on the jewelry scene today. The deep blue color of a sapphire ring is just right for an evening out.