Jasper Conglomerate

Sedimentary rock

Sedimentary rock (Conglomerate) with a chert/jasper clast

Also known as: Puddingstone (if matrix is fine-grained and clasts are well-rounded), Jasper Breccia (if clasts are angular)

Got a photo? Identify rocks instantly with the Rockby app

Open the app

Description

A conglomerate is a clastic sedimentary rock characterized by rounded to sub-rounded clasts larger than 2 mm in diameter, set within a finer-grained matrix (sand, silt, or clay) and cemented together. In the case of 'Conglomerate with Jasper Clast', a significant proportion of these gravel-sized clasts are composed of jasper. Jasper is an opaque, microcrystalline variety of quartz, typically red, yellow, brown, or green, due to the presence of iron oxides or other mineral inclusions. The overall appearance of the rock will be a heterogeneous mix of these colorful jasper clasts, along with other rock fragments (e.g., quartz, feldspar, granite, basalt) and mineral grains, all bound by the matrix and cement. The size, shape, and sorting of the clasts, as well as the composition of the matrix and cement, can vary widely depending on the depositional environment and source area.

How to Identify

Color: Highly variable, depending on the clasts, matrix, and cement. Jasper clasts will typically be red, yellow, brown, green, or mottled. The matrix can be grey, brown, red, or white. Overall, a multi-colored, speckled appearance.
Luster: Clasts (especially jasper) can have a dull to waxy luster. The overall rock luster is typically dull to earthy.
Texture: Clastic, coarse-grained. Characterized by visible, rounded to sub-rounded clasts greater than 2 mm in diameter, embedded in a finer-grained matrix. The clasts are typically poorly to moderately sorted.
Crystal Form: Not applicable for the rock as a whole. Individual jasper clasts are microcrystalline quartz, so no macroscopic crystal forms are visible. Other clasts will retain their original rock or mineral forms.
Cleavage: No overall cleavage for the rock. Individual clasts may exhibit fracture patterns (e.g., conchoidal fracture in jasper/chert), but the rock itself breaks irregularly along grain boundaries or through the matrix.
Geological Environment: High-energy depositional environments such as river channels, alluvial fans, glacial outwash plains, beaches, and submarine canyons. Requires a source area containing jasper or chert-rich rocks.

Key Facts

Hardness: Variable, depending on clasts and cement. Jasper clasts: 6.5-7 on Mohs scale. Overall rock: 3-7, depending on cement strength and clast composition.
Specific Gravity: Variable, typically 2.5-2.8 g/cm³.
Crystal System: Not applicable for the rock as a whole. Jasper (microcrystalline quartz) is trigonal.
Color: Highly variable, often multi-colored with prominent red, yellow, brown, or green jasper clasts.
Luster: Dull to earthy for the rock, waxy to dull for jasper clasts.
Transparency: Opaque for the rock and jasper clasts.
Fracture: Irregular to conchoidal (for jasper clasts).
Cleavage: None for the rock. Jasper clasts exhibit no cleavage.
Composition: Clasts: Predominantly jasper (microcrystalline quartz), but also other rock fragments (e.g., quartz, feldspar, granite, basalt, limestone) and mineral grains. Matrix: Sand, silt, clay. Cement: Quartz, calcite, iron oxides, clay minerals.

Quick Check

Color: Multi-colored, with prominent red, yellow, brown, or green clasts.
Luster: Dull to earthy overall, waxy on jasper clasts.
Streak: White (for quartz/jasper clasts), but the overall rock streak is not diagnostic due to mixed composition.

Physical Characteristics

Crystal Habit: Not applicable for the rock. Jasper clasts are anhedral, microcrystalline aggregates.
Cleavage Type: None.
Fracture Type: Irregular to sub-conchoidal for the rock; conchoidal for individual jasper clasts.
Tenacity: Brittle.
Luster Type: Dull to earthy (overall rock); waxy to dull (jasper clasts).

Formation

Conglomerates form from the lithification of gravel-sized clasts (particles larger than 2 mm in diameter). The presence of jasper clasts indicates a source area rich in chert or jasper, which are microcrystalline varieties of quartz (SiO2). These clasts are transported by high-energy currents (e.g., rivers, glacial meltwater, wave action) and deposited in environments such as river channels, alluvial fans, beaches, or submarine canyons. Over time, these sediments are buried, compacted, and cemented by minerals like calcite, quartz, or iron oxides, forming a solid rock. The jasper clasts themselves are typically derived from older sedimentary (e.g., banded iron formations, chert nodules in limestones) or volcanic rocks.

Usage

Primarily used as an aggregate in construction (road base, concrete filler) due to its durability. When jasper clasts are particularly colorful and abundant, it can be cut and polished for decorative purposes, such as facing stone, countertops, or ornamental objects. Some varieties, like the 'Puddingstone' type, are historically valued for their aesthetic appeal.

Age Distribution

Can form throughout geological time, from Precambrian to Cenozoic, wherever suitable source rocks and depositional environments exist.

Where to Find

Great Lakes Region, North America

Specifically, the 'Puddingstone' varieties found in Michigan (e.g., Drummond Island) and Ontario, Canada, are famous for their colorful jasper and chert clasts, often derived from Precambrian banded iron formations.

Western United States

Conglomerates with chert/jasper clasts are common in many sedimentary basins, particularly those associated with ancient river systems or uplifted mountain ranges where chert-bearing formations were exposed and eroded.

Australia

Various localities, especially in regions with ancient sedimentary sequences and banded iron formations, can yield conglomerates with jasper clasts.

United Kingdom

Certain 'Puddingstone' varieties, such as the Hertfordshire Puddingstone, contain chert clasts, though jasper may be less prominent than in North American examples.

Finding Tips

Look for High-Energy Environments

Search in areas that were historically or are currently associated with strong water currents, such as ancient riverbeds, alluvial fans, or coastal deposits. Modern river gravels can also provide clues to the presence of such rocks upstream.

Examine Outcrops and Road Cuts

Conglomerates are often well-exposed in road cuts, stream banks, and cliff faces. Look for rocks with a distinctive 'pebbly' texture and varied clast composition.

Identify Jasper Clasts

Focus on identifying the characteristic red, yellow, brown, or green opaque clasts within the conglomerate. These clasts will often stand out due to their color and hardness (scratching glass).

Check for Durability

Jasper is a very durable mineral. Conglomerates with abundant jasper clasts tend to be more resistant to weathering and erosion, often forming resistant ridges or caprocks.

Similar Rocks

Breccia

Also known as: Angular clastic rock

Sandstone

Also known as: Arenite

Chert

Also known as: Flint, Radiolarite

Quartzite

Also known as: Metamorphosed sandstone

Scientific Classification

Mineral Class: Sedimentary Rock (Clastic)
Group: Conglomerate
Crystal System: Not applicable for the rock. Jasper (quartz) is trigonal.
Chemical Formula: Variable, primarily SiO2 (from quartz/jasper clasts and cement), with other silicates, carbonates, and iron oxides.
Composition: Detrital fragments (clasts) of various rocks and minerals (including jasper) cemented together by a matrix of finer-grained material and chemical precipitates.

Explore Jasper Conglomerate

Identification Localities & Collecting Look-alikes

Identify rocks anywhere

Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.

Download on theApp Store Get it onGoogle Play Continue onWeb

Open in app