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. The birthstone of May, an emerald ring symbolizes the eternally returning spring. The velvety green color of emerald is a perfect match for spring and summer fashions.