Quartz-veined Mudstone or Argillite

Physical Properties

Hardness: 3-4 for the dark matrix, 7 for the white quartz vein. Color: Dark grey/charcoal with a white milky band. Luster: Dull/earthy for matrix, vitreous for vein. Crystal Structure: Fine-grained matrix, macrocrystalline vein. Specific Gravity: ~2.6.

Formation & Geological History

Formed from the lithification of fine silt and clay. The white band is a hydrothermal quartz vein, created when mineral-rich hot water filled a fracture in the solid rock and crystallized. Often found in orogenic (mountain-building) zones.

Uses & Applications

Primarily used as decorative garden stones or for geological study. Some high-contrast pieces are used in 'worry stones' or lapidary crafts.

Geological Facts

The appearance of a white line through a dark rock is often called a 'Wishing Stone' in folklore. The vein represents a moment of geological stress where the rock cracked and 'healed' with new mineral growth.

Field Identification & Locations

Identify by the high contrast between the dark, fine-grained host rock and the hard, glassy white vein. Found commonly in riverbeds and glacial tills. Use a hardness test: the white part will scratch glass, while the dark part likely will not.

Identified on: 5/2/2026

Mode: Standard