Obsidian

Arising in the wake of volcanic activity, obsidian is most commonly found buried at the base of long-dormant mountains. Although brittle, it naturally fractures into shear edges and smooth planes, a necessity for crafting knives and arrowheads in days long past.
  Description: Obsidian holds the appearance of a shard of glass, but with a natural shade ranging from deep purple to black. Smooth and reflective, obsidian will often display its contours with a glossy sheen.
  Creation: Crafting obsidian into an object that prominently features sharp edges takes half as much time.
  Natural Properties: Coming into sharp contact with an Obsidian object may cause a creature to take Id4 piercing or slashing damage. (This property does not increase the damage dealt by obsidian weapons, or allow obsidian armour to inflict return damage.) Weapons. Weapons made of Obsidian that deal either piercing and slashing damage gain the positive benefits of both damage types, and only suffer downsides that affect both damage types. Armour. When a creature makes a grapple attempt against a creature wearing obsidian armour, it takes Id4 slashing damage.
  Example Item: Obsidian Signet Ring. Unarmed strikes made with a hand wearing an obsidian signet ring deals Id4 piercing damage rather than their regular amount and type.