Obsidian

Obsidian is a type of volcanic glass, that is naturally formed when lava cools rapidly, causing crystal-like growth. In the Evernight series, obsidian is one of the materials that can repel wraiths.

Physical Appearance
Obsidian is typically black or very dark green in colour, with a slight sheen. It is hard and brittle, and can be crafted to have extremely sharp edges, or polished to a high gloss. There are some types of obsidian that have a slightly different appearance, most notably snowflake obsidian, which has white splotches that resemble snowflakes.

Uses
In the past, obsidian was often used to make tools or weapons, such as arrowheads or daggers. Notably, in Aztec society, obsidian was occasionally used to make knives used in human sacrifices. Obsidian was, and still is, used to make jewellry and decorative items as well.

In Evernight
Obsidian is one of the materials that can repel wraiths. Bianca Olivier's parents gift her with an obsidian pendant on a copper chain to keep wraiths at bay; she doesn't initially realise this, though, until Mrs Bethany explains it to her.