In very simple words Bentonite is a clay belonging to the smectite family mainly composed of montmorillonite and originated by volcanic ashes that sedimented into different areas and underwent hydrolyzation processes by different kinds of waters and consequently enriching in Ca or Na or Mg or K or H and so giving origin to Na+ Bentonite - natural sodium bentonite - or Ca++ Bentonite - calcium bentonite - or H+ bentonite - acid bentonite.
Bentonites vary in colour from grey to white, yellow, green, blue, black but more often yellow or yellowish green. The weather changes bring about a cellular characteristic like popcorn or a frothy structure around because of swelling and shrinking on wetting and drying. The bentonite has a waxy appearance whereas bentonites formed from volcanic ash gives it a hard structure.