Sunstone

Sunstone is a white cubic crystal that used to be used for navigation. It is only found in the northern fjords of Tamaris and was used extensively by early people in Retland. Sunstones are mined from the earth and are generally polished before use. They break into cubical shapes, though most remaining examples are quite small. It's normally clear, however, when held up against sunlight or moonlight, it changes color. The brightness and intensity of the color changes depending on the angle of the light and is effective both in cloudy weather and in twilight. Against the sun, the stone ranges between a dark amber to a pale yellow color while under moonlight, it turns purple. To an extent, it can be used to tell time by indicating which angle the sun or moon is at relative to a fixed position.
The people in Retland regularly traded sunstones, and examples of sunstones appear in many cultures. In the south, they were often inserted into gold sun-shaped pendants. They were viewed as gifts fit for royalty and many believed the mineral to have magical properties. Surprisingly, when exposed to magical light, the stone turns blue with light blue indicating a strong source of magical light. However, this effect has been largely forgotten with the loss of magic on Tamaris.
These days, the sunstone is believed to be mythical. The people in Retland have myths about how the gods gifted their ancestors with the sunstone and taught them to navigate, but no one has seen one for over a thousand years. A few alchemists have claimed to own sunstones, but those are unable to show the same abilities as real sunstones. The location of the sunstone mines is lost to history, and many people believe the stones never existed to begin with.