Piddock-bored Mudstone

Physical Properties

Hardness: 2-3 Mohs; Color: Grey to dark brown; Luster: Dull/Earthy; Structure: Fine-grained, non-crystalline; Features: Distinctive circular bored holes and pits.

Formation & Geological History

Formed from the lithification of silt and clay in marine environments. The holes are biological in origin, created by piddock clams (family Pholadidae) that burrow into the soft rock as they grow.

Uses & Applications

Educational specimens for geology and marine biology. Commonly used as 'hag stones' or lucky charms in folklore. No major industrial use.

Geological Facts

The holes are created by bivalves that use their sharp shells as drill bits. Because they cannot leave the hole once they grow, they spend their entire lives inside these rock burrows. These specific stones are often called 'hag stones' or 'adder stones' in various cultures.

Field Identification & Locations

Look for cylindrical, smooth-edged holes in soft sedimentary rocks along coastal beaches. They are frequently found in intertidal zones where piddocks are active.

Identified on: 5/3/2026

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