Necrite

A form of abyssal-influenced obsidian formed when regular stone (regular obsidian is easiest, but any stone can be transformed into necrite) is subjected to intense abyssal presence. It acts as an aether absorber, being especially capable at trapping and imprisoning jotnar and fey entities.

Physically, it's shiny black with an oily green sheen and feel. It always feels cold, regardless of the ambient temperature. It requires substantial magic to make it usable for anything, as it shatters easily.