The Heat Stone is a survival item for keeping players warm in Winter. It accumulates heat once near a fire, either placed on the ground, in a Chest, or carried in the player's inventory (including Backpacks). It requires 10 Stones, 1 Pickaxe, and 3 Flint to craft and an Alchemy Engine to prototype.
While having a charge, it emits warmth and delays the moment a character starts to freeze. It provides a maximum amount of 120 points of insulation. Additionally, as a heat source, its maximum temperature is 60C, and its five visual states correspond to the following temperature ranges, from coolest to hottest:

Freezing White : < 0C
Blue : 0C - 25C
Grey : 25C - 40C
Yellow :40C - 50C
Glowing Orange : 50C - 60C

A fully charged stone, along with providing a lot of warmth, also creates a small radius of light visible at dusk and night, preventing Charlie from attacking the player. The size of the fire affects the maximum charge the Stone can attain. The Heat Stone will also provide any given Chest the same small radius of light it provides the player when it is stored in said chest.