sedimentary

Jasper Conglomerate (Puddingstone)

Jasper Conglomerate (Siliceous Sedimentary Rock)

Hardness: 6.5–7 on Mohs scale; Color: Red, brown, or black jasper clasts embedded in a lighter cream/white/tan quartzite matrix; Luster: Dull to waxy; Structure: Clastic (coarse-grained); Cleavage: None (conchoidal fracture on clasts).

Hardness: 6
Luster: Dull to waxy

Identified March 22, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed roughly 2.3 billion years ago (Huronian Supergroup) during the Paleoproterozoic era. It consists of stream-rounded pebbles deposited in high-energy environments like riverbeds or alluvial fans, later cemented by silica under intense pressure.

Uses & applications

Used primarily as decorative stone, lapidary material for jewelry (cabochons), and ornamental landscaping rock. Historically used for making grinding stones or sharpeners due to high silica content.

Geological facts

Commonly known as 'Puddingstone' because the red pebbles in white matrix resemble raisins or suet in traditional British plum pudding. It is the unofficial state rock of Michigan (specifically found in the St. Joseph Island area).

Field identification & locations

Identify by looking for rounded pebbles (clasts) of varying colors distinctly 'glued' into a finer-grained matrix. Commonly found in the Great Lakes region of North America and parts of the UK. For collectors, look for high contrast between pebble colors and the matrix.

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