Glowstone

Glowstone, only mentioned in an extra page at the end of Chapter 01, is a mineral found in The Abyss. When certain frequencies are applied to a polished glowstone, it will produce light. It doesn't produce heat making it an ideal light source; but, when under pressure, explodes easily.

Uses
Delvers' Headlamps - Appears to be a combination of the gem, an iron can, airtight glass container, a wire, a processed talon crystal, and some water.