Snowgift Brooch
This ironoak brooch is carved with a snowflake; in direct sunlight it is damp, out of it, frosted.
I confirmed this anonymously donated work via exhaustive experimentation: the wearer can touch anyone else to render them immune to all cold until the next sunrise, turning one of the six points of the snowflake to iridescent ice. If all six points turn to ice, the wearer becomes immune too.