[Bernard Marx]

The crystal in question was iolite, aka cordierite after the name of the mineralogist who first described it. This mineral is trichroic (offering 3 different colors when viewed in different orientations). Two of them, blue and yellow, are very distinct.

The modern usage of the word “sunstone” refers to an entirely different mineral, varieties of feldspar.

Here are two links that give more detail about how early seafarers might have used iolite crystals as a navigation aid.

Navigation and Iolite

[Jedi Master Pikachu]

Appreciated.