Facets is the single largest collection of core extension methods and standard class additions available for the Ruby programming language. Premium Core Extensions and Standard Additions. Facets is a large collection of core extension methods and standard additions for the Ruby programming language. The core extensions are unique by virtue of thier atomicity. Methods are stored in small, tightly related collections, allowing for highly granular control of requirements. The modules include a variety of useful classes, mixins and microframeworks, from the Functor to a complete Annotations system.