Conglomerate with Native Copper

Physical Properties

Hardness: variable matrix (3-5), metal inclusions (2.5-3); Color: dark grey to black matrix with reddish-metallic copper; Luster: earthy/dull with metallic inclusions; Structure: clastic/coarse-grained fragments.

Formation & Geological History

Formed through the deposition of rounded rock fragments in a high-energy environment, followed by the precipitation of native copper from hydrothermal fluids filling the voids between clasts, typical of the Keweenawan Rift (approx. 1.1 billion years ago).

Uses & Applications

Historical source of copper ore; currently valued as lapidary material, collector specimens, and decorative 'pudding stone' items.

Geological Facts

This specific type of rock is famous from the Keweenaw Peninsula of Michigan, the only place on Earth where large-scale commercial mining of native copper occurred within conglomerate and basalt rocks.

Field Identification & Locations

Identify by looking for rounded pebbles (clasts) cemented in a darker matrix with visible metallic copper seams or blebs. Found in glacial drift and copper-bearing districts of Upper Michigan.

Identified on: 4/21/2026

Mode: Standard