this stone even works if its cloudy. there are 2 lightbeams which you can see inside the stone and the brightness of this 2 lightbeams is used to see where you need to travel to get to specific positions. you can use this stone even 40 minutes after the sun is not visible anymore. it's a cool thing..
The revelation that the mineral was used for such a purpose is the special news. The actual mineral itself isn't all that rare and its properties are well known documented.
https://en.wikipedia.org/wiki/Sunstone_(medieval)
http://rsos.royalsocietypublishing.org/content/5/4/172187
https://m.bulbapedia.bulbagarden.net/wiki/Sun_Stone
is this it?
*pokemon theme*