Mercury is the natural ruler of the sign of Gemini, so it is at home in this sign. Mercury in Gemini indicates a lightning fast mind with can grasp new concepts almost instantaneously. They are often highly intelligent and eloquent individuals. Furthermore since Gemini is an Air sign it gives a great ability to communicate ideas and concepts. People with Mercury in Gemini can be great speakers and will never be short of a word. The problem with this combination can be an interest and fascination and superficial knowledge in many subjects but not an ability to settle and specialize and deepen knowledge in one area in particular. The reason for this generally is the natives soon lose interest in a subject as a newer more interesting concept turns up.