I was most impressed by the calcite sunstone. Tremendously useful for navigation.

That being said, one would have to find which way is “straight” which may not be perpendicular to one of the crystal’s faces, depending on how light refracts and is polarized through it.

One could probably do that with a small bright light source, like a candle through a pinhole, build a wood holder that indicates what direction the light particles go straight through the crystal without being polarized, and use that to calculate the sun’s angle.

Difficult but doable, I guess.