Complex sentence: The carat is a unit of mass used for measuring gems and pearls, and is exactly 200 "mg" (0.007,055 "oz", 3.086 grains).
Split into simpler sentences: The carat is a unit of mass used for measuring gems and pearls. Nowadays a carat is defined as exactly 200 "mg" (0.007,055 "oz", 3.086 grains).