Bentonite clay or, more correctly, sodium bentonite, is an absorbent clay that swells when it gets wet. It also gets slick and stringy. My first experience with bentonite was as a summer student working at a coal mine. We drilled several exploration core holes to define a new lease. The holes were sealed with bentonite chips which would expand enough to seal the hole when they got wet. The chip seal was so important that the BLM sent two representatives to observe the process and the director of technical services filled the hole personally. Apparently, they were all afraid that summer students would feed the chips too fast and jam the hole too near the collar and not allow the remainder of the chips to form the seal.
Bentonite is also used in drilling fluids. It is added to the water being pumped through the steel and helps to lift the cuttings to the surface as well as seal the sides of the hole to maintain the sides to be water tight. When I was in Africa on a coalbed methane exploration project the drillers were experimenting with some additives in the drill water. After about a day of drilling they were surprised to start getting water back. I realize that Africa is dry but the drillers shouldn't be that surprised to be getting fluid back.
The medical industry uses bentonite for its ability to absorb water and other toxins. Bentonite is used to treat things such as constipation, bloating and gas, iritable bowel syndrome and poison ivy.
One of the biggest uses of bentonite in the mining industry is as a liner. The clay swells so much when wet that when the top few inches become saturated they form a water tight seal. Bentonite clay liners are used for leach pads, tailings impoundments and water ponds because they are so much cheaper than plastic liners.
I also saw an interesting application of bentonite on the Discovery Chanel. On their show 'Dirty Jobs' Mike Row and the crew were harvesting walleye fish eggs and delivering them to the New Mexico Department of Game and Fish hatcheries. After the eggs had been inseminated they coated them with a solution of bentonite and water. I have been unable to find an explanation for this but I can only assume that it is to protect the eggs from being squashed during the rest of their handling.
I was pleased to get some rock samples from my father in law and was impressed by the variety in color. I was familiar with the dark brown color, but had no idea that it came in shades of green.