Dragonglass

Dragonglass, known to the maesters as obsidian and to the children of the forest as frozen fire, is a volcanic glass with a singular and vital property in the world of A Song of Ice and Fire: it is one of the few substances known to kill the Others, the ancient cold creatures of the far north.

Properties

Dragonglass is black, glassy stone formed where fire and rock meet, brittle yet capable of holding an edge sharper than the finest steel. Blades and arrowheads of dragonglass shatter easily but cut cleanly. Its great significance is that a dragonglass weapon can slay an Other, which dissolves into cold mist when struck, where ordinary steel is useless against them.

Sources

The richest source of dragonglass in Westeros is Dragonstone, the volcanic island seat of House Targaryen, beneath which the substance is said to lie in abundance. In ancient days the children of the forest used dragonglass blades and gave dragonglass weapons to the Night's Watch, a custom long forgotten.

Importance to the Watch

As the threat of the Others returns, the value of dragonglass is rediscovered. A cache of dragonglass weapons and a horn is found buried at the Fist of the First Men, and the rangers of the Night's Watch come to understand that this ancient stone may be the realm's best defense against the coming Long Night.