A skilled Blacksmith can coat or forge Weapons and Armor out of different Materials to gain different benefits. A Weapon or Armor must be made of the same type of Material (meaning you cannot forge a Leather Adamantine Armor).
Price is added on top of what the Base Equipment normally costs: an Iron Longsword is 15g, a Silver Longsword is 15+75 = 90g.
Silver
This precious and pretty metal, while commonly used as payment, is known for its purity and ability to undercut supernatural defenses.
Also used in religious Rituals and divine magic.
Average Price: +75g
Weapons: Silvered Weapons can cut through the resistances of supernatural foes, such as Werewolves and Ghosts.
Armor: Wearing silvered armor provides advantage on saving throws against disease and being poisoned, as well as ghostly possession.
Mithral
This blue-silvery, shiny metal offers protection comparable to iron while being significantly thinner and lighter. Mithral chain can be worn under normal clothing without being very noticeable to the wearer or others.
Also a favorite among jewellers, its often made into detailed jewelry and decorations that would be too fine for normal metals.
Average Price: +150g
Weapons: Melee weapons made from Mithral that aren't Heavy become Light.
Mithral Thrown Weapons double their throwing range.
A small creature can wield a Heavy weapon made from Mithral without disadvantage on attack rolls.
Armor: If an Armor normally imposes disadvantage on Dexterity (Stealth) checks or has a strength requirement, the mithral version of the armor doesn’t.
Adamantine
An exceptionally hard, durable and heavy metal, appearing darker than iron, with a deep green sheen. It doesnt bend at all until it shatters.
Adamantine Blades never dull, even if theyre struck against rocks constantly.
Average Price: +300g
Weapons: Melee weapons of adamantine are unusually effective when used to break, cut or pierce objects. Whenever an adamantine weapon hits an object, the hit is a critical hit. Can overcome physical resistances of Stone creatures or similar.
Armor: While you're wearing Armor reinforced with Adamantine, any critical hit against you becomes a normal hit. If your Strength is below 13, Adamantine Armor reduces your Speed by 10ft.
Obsidian
This volcanic glass is deep black and sometimes shimmers in purple and blue, and is usually found in jagged shapes. Normally extremely brittle, Merfolk artisans managed to temper it into a somewhat more durable material.
Obsidian makes for terrifying serrated weapons capable of delivering brutal strikes. It doesnt lend itself to making Armor.
Average Price: +100g
Weapons: Critical Hits with Obsidian Weapons add another bonus weapon die with maximum damage. (→ A d12 Greataxe would add +12 Damage.)