Permafrost (3.5e Variant Rule)
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Permafrost is a state in which layers of ice have formed, each layer remaining below the freezing point −90 °C (−130 °F; 180.0 K) for two or more years, though normal freezing points for water begin at 0 °C (32 °F). The layers meld into one another over time, though the longer the process has time to develop the more strength can be found in the thick compact ice. Most craftsman who deal with frozen permanency allow permafrost material to harden over decades, having special facilities built in order to obtain perfect specimens of permafrost that must be harvested by Ice Elementals who are also responsible for tending to the facility's temperatures and conditioning. Permafrost normally cannot be melted by temperatures lower than 70° (160 °F), being immune to fire-based spells (except for examples such as Meteor Swarm as though the fire wouldn't melt the weapon, the force alone would destroy the weapon on impact) and dispelling-magic-based spells, but is perfectly capable of being destroyed with enough force depending on the thickness of permafrost ice.
Objects encased or made from Permafrost are granted Hardness equal to the inches of thickness, starting at 12-inch thick with 10 Hardness, with a maximum being 10ft-thick permafrost granting 100 Hardness. There are specimens of Permafrost found to having a higher thickness and hardness, but everything past 10ft is too strong to break by most magical means and is impossible to be used for crafting as Permafrost of higher thickness contains levels of coldness that causes Hypothermia and Frostbite (except of those born from the Elemental Plane of Fire) to creatures upon touch and is treated as an Extreme Cold Condition.
Reference: Frostburn, pg 81 "Blue Ice"