Viking Sunstone found in ship wreckage! Calcite crystals like Icelandic Spar create a double image, splitting light into two rays. If the crystal is held east-west, the double image becomes a single image and thus allows a sailor to locate the sun. The crystal's refractive qualities continue to be useful even in low light, on a cloudy day, or for a while after the sun has set.
Iolite. Known as the gemstone of the Vikings, Iolite was used by Norse and Viking explorers as a navigational aid, as a properly oriented stone would change color in relation to the direction of sunlight, allowing for crude navigation in the fog- enshrouded northern Atlantic waters where the direction of the sun was otherwise impossible to discern. https://www.etsy.com/listing/126884990/sterling-silver-ring-with-koi-fish
Azurite: Dense merged crystals of Azurite are found in copper-ore areas of USA, in Russia (Altai and Ural), Kazakhstan, Germany, Zaire (Africa) and Australia. Separate large crystals of this mineral are found more rarely. From the point of chemistry azurite is a copper carbonate. Its blue color is explained by iron compounds.
Fluorite (CaF2) Blue Fluorite with Phantom Purple - Macro view of a very small but special fluorite crystal speciment, only 1 1/4" tall, captured through the glass display case. This shows a clear blue outer row of translucent blue cubes grown over an initial purple crystal.from cobalt123