This is a large, single Mercenaria clam in a dark gray micritic limestone matrix, with golden yellow calcite crystals to 2 cm long filling the upper half of the specimen. The calcite is fluorescent blue-green under shortwave and longwave UV light (see photo), and even phosphoresces for a few seconds when the UV light is extinguished. These specimens are Plio–Pleistocene in age (1–2 million years old).