This isn't my idea. I wish I knew who to give the proper credit to. One of my fellow guild members (Janice V) was using her nifty tool and she told us how to make it,the next month another member (Barb B) brought in a little bag for each of us filled with everything we needed to make our own so I went home and did just that.
It's quite simple to make. I guess you could make it any shape you wanted to but this shape is easy to hang on to.
Grab some scrap clay, roll it into your preferred shape and then simply roll it in a pile of different sized/shaped salts,sugars,sprinkles or even little beads. Bake and voila! An instant texture tool.