According to Wikipedia, tourmaline is a crystal silicate mineral combined with other elements to produce different colors. This variety is called watermelon since it has the colors and look of watermelon slice. The compounds that make it up could be elbaite (green=verdelite) or liddicoatlite.
These specimens are from Brazil and are on display at the Indiana University Southeast (IUS) in New Albany Life Sciences Building in the Geosciences department. They were part of the Jimmy C. Davis collection.