Lightstone

Lightstones were glowing yellow or blue-colored 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. Once, the Lightstone mine was cut off from the workers causing Onu-Koro to come to a stand-still because no one could see. Oil lamps were used during the crisis. Only after the mine was reachable could work continue.