didn't see a good way to shorten.

1) crystals with an interesting polarised effect which allow you to identify the vertical plane of the sun in cloudy skies, and therefore effect navigation at sea in overcast daylight, modulo some basic idea of the time.

2) quasi religious carved stones, "sacrificed" by burial, in attempts to placate the gods over the awful horrors of a post-eruption landscape and climate.

The article is about the second kind.