Gemstone means "gemstone" and the new family of EFM32s are named after gemstones and are collectively referred to as EFM32 Gemstones.
As of April 2016, two families, EFM32PG and EFM32JG, have been released, EFM32Pearl Gecko is pearl and EFM32 Jade Gecko is jade.

Silicon Labs documentation and community forums sometimes use the term Gemstone to refer to EFM32PG, EFM32JG, etc.