I recently started designing and making jewelry. I'm particularly fascinated by Iolite. [Iolite]("https://en.wikipedia.org/wiki/Cordierite") is commonly referred to as "water sapphire" or "viking's compass". This is because it changes in tone depending on it's angle to the sun. Historically it would have been useful because the effect works regardless of cloud cover and even when the sun is below the horizon. I'm not sure if the nickname is a historic remainder or attached during modernity (the jewelry industry is both ancient and opaque) but the effect is real, and could potentially be of concrete utility during navigation.