This page contains rule updates and additions for special materials usable in game. Some of these materials are suitable for use in weapons or armor, and some are simply intended for crafting purposes or have a novel effect of their own.
Mithral is too potent a material, and its effect doesn't make sense: heavy armor is heavy because it restricts movement, not because of its weight. Thus, mithral no longer makes heavy armor into medium or medium into light. It does continue to reduce a suit of armor's check penalty and arcane spell failure chance, as well as its weight, as described in the DMG. Also, a character who wears medium mithral armor is treated as though he were wearing light armor for purposes of move rate.
An exception: Elven chain is iconic, and is therefore actually treated as light armor.
Obsidian is a black, glass-like material that originates from volcanic flows. It is relatively fragile, even compared to other stone, but it sharpens to an edge like no other. This property is useful for creating weapons capable of inflicting deep wounds.
Any light slashing or piercing weapon, as well as spears, axes, bolts, and arrows, can be crafted from obsidian. Such weapons cost five times their standard value in addition to the masterwork cost. Obsidian weapons break on a natural 1 attack roll, but have their threat range improved by 1. This threat range increase stacks with other effects that improve a weapon's threat range.