There is a widespread area that spans a region within America's Heartland between northwestern Nebraska to southwestern South Dakota that produces a unique and beautiful fluorescent material known as Watermelon Rock. Watermelon rock consists of red/orange fluorescent calcite, sandwiched between two layers of bright green fluorescent chalcedony. The name for this material comes from its similarity to the coloration of a watermelon, with its bright red interior surrounded by a green rind. The brightest fluorescent response typically occurs under short wave UV, but some specimens display best under mid wave. The example shown here was collected from Buffalo Gap National Grasslands, near Weta, South Dakota, but similar material is also found commonly around the town of Crawford, Nebraska.
Colorful watermelon rock, seen in the above photograph under short wave UV (254nm). A seam of red/orange fluorescent calcite is sandwiched between two layers of uranyl-activated green fluorescent chalcedony.
Same specimen seen under visible light. This relatively flat specimen consists of a layer of pale yellow calcite sandwiched between two layers of light grey chalcedony. The size of this piece is 120 x 66 x 18 mm, and it weighs 196 grams.