5.1" Fluorescent, Yellow Calcite Crystal Cluster - South Dakota

Elk Creek, South Dakota is renowned for producing these calcite and barite specimens. The barite crystals collected from here are considered some of the nicest specimens around the world and can sell for thousands of dollars. These mineral specimens form as fossiliferous septarian, calcareous concretions that erode out of the Gregory Member of the Pierre Shale. These concretions have to be carefully opened with specialized diamond saws. If opened incorrectly or too hastily, the calcite and world-class barite crystals can be damaged easily.

About Calcite Crystals

Calcite crystals are a form of calcium carbonate (CaCO₃) known for their diverse shapes, transparency, and vibrant range of colors. They typically form in rhombohedral, scalenohedral, or prismatic shapes, often with well-defined, sharp edges and glossy surfaces. Calcite crystals are often translucent or transparent, sometimes displaying a double refraction effect where objects viewed through the crystal appear doubled. They can appear in various colors—white, clear, yellow, pink, blue, green, and orange—depending on impurities or trace minerals.

A notable characteristic of calcite is its reaction with weak acids like vinegar, which causes it to effervesce, or fizz, as it releases carbon dioxide. This property makes calcite crystals a key tool in geological identification and studies. Calcite forms in many environments, from sedimentary rocks like limestone and marble to hydrothermal veins.