Libby was a teenage girl who was Sheldon Cooper's first crush. They met in 1990, when she was 17 and he was 9.
Libby helped Sheldon gain an interest in geology by introducing him to rocks that can shine bright diverse array of colors in the dark, helping him realize that tectonic plates are what helped lead to a diversity of animals upon different continents to the point many exotic animal species are only endemic to one continent. Sheldon was intrigued by this and wanting to learn more to attack Pastor Jeff's endorsement of religion and Christianity, he joined Libby on a journey to study more rocks. However he overheard her claim that despite his high IQ she still sees him as grade school kid crushing thus stopping future interactions between them. Eventually Sheldon would come to hate geology.