stone bathtub
A bathtub made of stone, often found in traditional Japanese settings or hot spring resorts.
steam bath in a stone chamber
A steam bath taken inside a natural or man-made cavern or stone chamber, historically used for therapeutic purposes.
The inn's open-air bath had a large stone bathtub.
It is said that people in this area used to cure illnesses by taking steam baths in stone chambers.
Compound of 石 (ishi, 'stone') and 風呂 (furo, 'bath'). The exact historical origin of the steam bath sense is uncertain, but it likely refers to the use of stone chambers for bathing.