Bentonite
Details
When it comes to oil-absorbing clay masks, bentonite will probably be one of the first ingredients on the INCI list. Technically bentonite clay is mostly montmorillonite + something else, and thanks to the something else bit, bentonite comes in different types and colors.
The color depends on the mineral content of the clay: white bentonite is rich in boron and fluoride, yellow is rich in manganese and zinc, green is rich in copper, zinc, and manganese and the pink clay is rich in boron.
No matter the color, bentonite is excellent at absorbing things: it can suck up the sebum and gunk from the skin and make it instantly smooth and matte. Not only that, but bentonite has a negative ionic charge and thus can attract things with a positive charge. Things with a positive charge include bad bacteria and toxins and bentonite clay masks can help to clear those out of the skin and pores (btw, bentonite is edible and has the same detoxifying effect internally).
Thanks to bentonite's effect against bad bacteria and pathogens, there is also some research showing that bentonite can help to calm skin infections, soothe skin allergies and might work for skin conditions like psoriasis and eczema.
On the other hand, the downside of bentonite being such a good absorbent is that it can suck up more than the excess sebum and used too often, it can easily dry out the skin. So use it for good measure, and never forget to moisturize afterwards.