Chert Pebble / Brachiopod Fossil Internal Mold

Hardness: 7 (Mohs scale), Color: tan, grey, brown, Luster: waxy to vitreous, Crystal structure: cryptocrystalline (microscopic quartz), Cleavage: none, fracture: conchoidal, Specific Gravity: 2.65

Formed through the accumulation of silica-rich marine organisms or chemical precipitation within limestone beds. This specific specimen appears to be an internal mold/steinkern of a brachiopod shell that has been weathered into a stream-worn pebble. Likely Paleozoic or Mesozoic era (approx. 66–541 million years old).

Mainly of interest to amateur paleontologists and rockhounds; historically used for stone tools (if high-quality chert); modern uses include gravel and landscaping.

Because chert is harder than the limestone it usually forms in, it survives erosion better. Internal molds like this occur when sediment fills a shell, hardens, and the original shell material later dissolves, leaving a 'stone heart' shape.

Identify in the field by the 'heart' or 'nut' shape which is symmetrical across one plane, the conchoidal (shell-like) fracture patterns on broken edges, and its inability to be scratched by a steel knife. Commonly found in riverbeds and areas with exposed limestone or chalk.