Cernit Number One is a premium modeling clay produced in Belgium by The Clay and Paint Factory. The Number One line of Cernit polymer clay is opaque, and is stiffer than the other lines, especially the Cernit Translucent and Cernit Metallic. It is similar in consistency to to Sculpey Premo.

The 500g bars are a good savings over buying individual 56g bars - you get almost 9 bars worth of clay for much less!

A great overview of the Cernit lines of polymer clay can be found here - Review of Cernit Polymer Clay