Sometimes conditions in the real world lead to solutions in the fantasy world.
Stone of Warmth
A stone of warmth, if kept on one's person such as in a pocket or a pouch, will keep one comfortably insulated from even the bitterest cold. If held in one's hand, it will extend the zone of protective warmth in a 4 cubit radius.* If held in both hands and rubbed, it will generate enough heat to melt ice rapidly at a distance of 8 cubits.** This third function will cause 1d8 hit points of damage to creatures that are especially vulnerable to heat. A successful saving throw vs. magic will reduce the damage to one half.
Like other magic stones, a stone of warmth has no standard size, shape, or coloration, and will only activate when in close proximity to a living creature.
* 6 feet or 2 m.
** 12 feet or 4 m.