The
Crystal family of methodologies views software development as a "cooperative game of invention and communication, with a primary goal of delivering working useful software and a secondary goal of setting up the next game". Crystal is both people and communication centric, and highly tolerant (is shaped per project).
Crystal has seven core properties: Frequent Delivery, Reflective Improvement, Osmotic Communication, Personal Safety, Focus, Expert Users, Automated Tests, Configuration Management, and Frequent Integration.
Crystal teams choose practices to meet those properties, with Crystal documenting practices that have proved successful in other projects but not mandating them. Crystal requires incremental development with reflection on those iterations. The purpose of those reflections is to shape the practices the team us using.
Crystal Clear is the lightest methodology in the family, suitable for co-located teams of up to 10 people. It is a lower-discipline methodology than XP (1st edition) but also requires some documentation.
Crystal Yellow is for teams of up to 20, Orange for up to 40 and so on. As team size increases, the option to use co-location and osmosis to communicate diminishes, so the team must introduce compensating practices (such as Scrums: Scrum of Scrums).