Material Hardness
(or Material HD Rule)
Hardness
All materials fall into one of five levels of hardness, expressed as a die (d4 up to d12). Core Materials will only be in the first three levels, unless enhanced by a Specific Monster Material or a Material Template. The die of each level represents the HP of items made from it, as well as the die rolled to damage it. For example, a steel blade is hardness d6, so it's HP is 6 and it rolls 1d6 to determine damage against it.
Hardness Modifiers
Individual materials within a level of hardness may vary in exact durability, expressed as a modifier to it's hardness die. This modifier does not effect it's HP, but instead modifies it's roll to determine damage against it. Regardless of modifiers, damage is always at least 1. For example, a bronze sword has a hardness expressed as d6+1. This means it's HP is 6, and it rolls 1d6+1 to determine damage against it. Modifiers from multiple sources, such as material and magic, do stack unless otherwise stated.
When to roll hardness damage...
There are three instances when a player will need to roll hardness damage.
Natural 1 attack roll means the character missed the target in a way that damaged the weapon, requiring a hardness damage roll.
Opponent rolled a Natural 20 attack roll means the blow from the opponent may have damaged the character's armor, requiring a hardness damage roll. This is up to the DM.
Directly attacking something of an equal or higher hardness level will result in damage. If the target and attacking weapon are of equal hardness level, both must make a hardness damage roll. Otherwise, only the weaker item must make the hardness damage roll. This does not include combat, as there are other factors at play that would reduce the odds of weapon or armor damage, thus the first two listed reasons for hardness damage.
When HP Reaches 0
Damage on an item is in the form of wear and tear. Cracks on a sword. Holes in leather. Chinks in armor. But, if an item reaches zero hit points, it breaks or otherwise falls apart.
Recovering Hardness Damage
There are two ways to restore the HP of a damaged item.
A smith, fletcher, etc can repair the damage. Mundane items usually take about a day to repair, and depending on the item and the person repairing it, costs between 2 to 5 gp for labor and materials. Items made from exotic materials may require more of the material in order to do repairs. For example, a Celestrium blade will likely require some Celestrium ore to do major repairs.
Completely broken items (Items that lost all their HP) will require twice the time and repair costs, including sample materials.
The Mending Spell can restore a number of hit points to an item equal to the spell slot used to cast +2. So cast normally as a cantrip, the spell restores 2 hit point to a damaged item. This works regardless of the materials and can be used to restore broken items (items that lost all their HP).
Magic and Hardness
A magically enchanted item is embued with a magical force that adds to the protection of an item. The rarity of a magic item is a representation of the item's magical power. Common magic items have standard hardness based on their material. Uncommon have a -1 Hardness Modifier, Rare have a -2 Hardness Modifier, and Very Rare have a -3 Hardness Modifier. Legendary magic items have 1 higher hardness level and a -3 Hardness Modifier. These modifiers stack with any from the material. Relics are the same as Legendary, but still can only be destroyed in the ways cited in their listing.
Damaged magic items retain their magic, but may act oddly or randomly spark. They must be repaired through the use of the Mending spell.
Broken magic items lose all magic essence in 3d6 days. In order to be repaired, they must be repaired completely and magically before that essence is gone. Only the Mending spell cast at the appropriate spell level to fully restore the Hit Points of the item will work to make the item whole and magical again. So a Mithral magic sword would require a 6th level Mending spell to be restored and retain it's magic. But magic is a fickle thing, and some items have special requirements for repair, and other are simply lost forever if destroyed.