also: こうみょうせき
noun
kōmeiseki; kōmei stone; slightly radioactive stone used in hot springs and traditional medicine
A mineral stone, often containing radium or other trace radioactive elements, historically placed in baths or used in folk remedies for purported health benefits. The term is primarily encountered in contexts related to onsen and traditional wellness products.
この温泉では、浴槽に光明石が使われている。
At this hot spring, kōmei stones are used in the baths.
光明石は微量の放射線を出すことで知られている。
Kōmei stone is known for emitting trace amounts of radiation.
The name 光明石 (kōmeiseki) literally means 'bright light stone,' likely referring to the stone's historical association with radium's glow or its perceived health-giving radiance. The exact origin of the term is uncertain, but it became popular in early 20th-century Japan alongside the radium craze.