heated stone; warming stone
A stone heated and wrapped in cloth, placed against the body for warmth; a traditional practice, now mostly historical or in folk contexts.
Travelers in the old days kept a heated stone inside their kimono to ward off the cold.
chrysotile
Standard kanji spelling for this traditional warming stone.
Compound of 温 (warm) and 石 (stone). The reading おんじゃく is the on'yomi of the characters.