sedimentary

Jasper Conglomerate (Puddingstone)

Siliceous Conglomerate (Quartz and Jasper clasts in a fine-grained matrix)

Hardness: 6.5-7 (Mohs scale), Color: Grey or cream matrix with distinctive red/brown jasper pebbles, Luster: Dull to waxy, Crystal Structure: Microcrystalline (Cryptocrystalline quartz), Cleavage: None/Conchoidal fracture, Specific Gravity: 2.6-2.7

Hardness: 6

Identified March 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed roughly 2.3 billion years ago during the Huronian Supergroup period. It is composed of river-worn pebbles of jasper and quartz that were cemented together by a fine-grained sand/silica matrix and later subjected to intense pressure.

Uses & applications

Used primarily as decorative stone, lapidary material for jewelry (cabochons), and as ornamental garden stones. Historically used by Indigenous peoples for making tools due to its high silica content.

Geological facts

Commonly known as 'Puddingstone' because the red pebbles in the light matrix resemble raisins or plums in a suet pudding. It is the unofficial state rock of Michigan (specifically the St. Joseph Island variety).

Field identification & locations

Identify in the field by looking for rounded, colorful pebbles (usually red jasper) firmly embedded in a harder, lighter-colored sedimentary or metamorphic matrix. Commonly found on beaches of the Great Lakes (especially Lake Huron) and in glacial till across the Midwest.

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