I did an online search as well, and even though I would have supposed it was made of alumina from the shape, the Dipper is not in the Alumina section of their website, but is with the Plugs. There is a small silicone plug video associated with the Dipper on the Tantus' website and its price is half that of the Pace.
So, since they also mention to use water-based lube, I'll go with silicone.
The Dipper is made of our 100% Ultra Premium Platinum Silicone. It is a firmer silicone than our other plugs, so you can really feel the ridges. I love the nice long neck, it really stays put and the round ball doesn't get in the way during intercourse.
Unless you keep a toy in for a long time there shouldn't be much mess usually, provided that you're of regular habits, etc.
If there is some mess 10 or so minutes soaking helps quite a bit. If the thought of using a sink is unpleasant you can always tie a string around the plug and let it dangle in the toilet for a while. After that a quick wash, sanitize, and you're good to go.