Rock

[sam1ellie]

Absorbent clay like rock used for cat litter?

??????? ?????

[New Forester]

It's fullers' earth.

 

Also used as drilling mud in oil wells etc.

 

(Note that it is named after the occupation of fulling -- treating cloth to soften it -- not a chap called Fuller, so it doesn't get a capital initial.)

[Clanad]

New Forester is essentially correct.  However Fuller's Earth and drilling muds are different in that drilling mud is made from a naturally occurring mineral called bentonite.  Bentonite is mined in the western US and consists primarily of montmorrilonite.  This aids in the "clumping" of the litter you see advertised...