Carbon Core Guard Structure. The inner case is made of fine resin that incorporates carbon fiber for light weight along with rigidity that is superior to that of resin. This material protects the internal module from shock-induced damage and deformation. The outer case is made of stainless steel. The dual back cover protects the back surface against impact. The smaller case of the GSTB200 series provides a comfortable fit, even though the watch is equipped with a wide face. The light-on-dark LCD contributes to both functionality and face design.