Bentonite is a type of clay whose main component is montmorillonite, a clay mineral composed of very fine crystals.
Bentonite is made from ash and lava (volcanic ash) that was ejected by volcanic eruptions a great long time ago, 20 million years ago (during the Miocene epoch of the Cenozoic era).
It is believed that volcanic ash deposited on the seabed or lakebed is transformed into bentonite by two processes: "diagenesis" (the process that occurs to sediments after they have settled) caused by geothermal heat, geological pressure, and seawater, and "hydrothermal action" caused by eruptions from underground.