I have used the Rivesto Marmorino products, and have found them pretty good for the most part. The product is all natural and has been fairly consistent in quality each time I have used it for various Venetian plaster techniques. (there are several techniques)It only comes in two grades of grains though, which means that some of the Venetian Plaster techniques cannot be applied in the truest sense, but in this modern era, clients who want it don't seem to care about the technique as much as the finished result. Buy a 5 gal container and play around with it.

Ciao
Carter