Lightstone

Lightstones were glowing yellow, and sometimes blue, glowing crystals that grew in caves and were mined by Matoran, specifically Onu-Matoran. They were used for lighting up dark places, most notably the tunnels of Onu-Koro and Onu-Metru.

One time on Mata Nui, the Lightstone mine was cut off from the workers, thus causing Onu-Koro work to come to a slow crawl because no one could see. Oil lamps were instead used during the crisis. Later, the mine was reachable again and work continued.

Valokivi