Opal engagement rings? Absolutely! Diamonds may be a girl's best friend, but Opal engagement rings are a unique, original, and a far more affordable choice. This birthstone of October has a luminosity and glow that makes an opal the treasure that it really is. The rainbow of colors on a neutral white background makes an opal ring a perfect match for any outfit. Black and gold make a beautiful and elegant combination. A black onyx ring is a perfect accessory for any outfit.