Watermelon Calcite, Buffalo Gap Grasslands, South Dakota
Contributed by: Chris Clemens
minID: G95-5Y3
Date: Apr 15th, 2026
Locality: Buffalo Gap National Grasslands, South Dakota, USA (See on Mindat)
Size: 6.6 x 1.8 x 12 cm
Weight: 196 g
Description:
This material is known as "watermelon calcite" or "watermelon rock" due to its colorful red and green, fluorescent response under UV. This flat specimen consists of pale-yellow calcite sandwiched between two layers of light grey chalcedony. Under shortwave UV the chalcedony fluoresces green and the calcite layer red/orange, giving the appearance of a succulent piece of watermelon. This specimen was collected from the Buffalo Gap Grasslands, near Weta, in Jackson County, South Dakota.
Under shortwave UV (254 nm), the middle calcite layer fluoresces orange/red, sandwiched between two layers of green, fluorescent chalcedony. The green fluorescence of the chalcedony layers is activated by trace amounts of uranium (in the form of the uranyl ion).
Under visible light, this flat specimen consists of pale-yellow calcite sandwiched between two layers of light grey chalcedony.
Originally posted by Chris Clemens on Nature's Rainbows in 2024.
Summary of luminescence responses:
Calcite (Mindat) (RRUFF)
- Fluorescence under Shortwave (254nm Lamp/Mercury) UV light: Red
- Fluorescence under Shortwave (254nm Lamp/Mercury) UV light: Green