Breccia
Coarse clastic rock (angular clasts + matrix cement)
A coarse rock of angular, sharp-edged fragments cemented in a matrix, marking nearby rockfall, faulting, or impact.
- Mohs hardness
- Variable, ~3-7 depending on clasts and cement
- Color
- Variable; mixed clast and matrix colors
- Type
- sedimentary
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Overview
Breccia is a coarse-grained rock made of angular, sharp-edged clasts (larger than 2 mm) cemented together in a finer-grained matrix. The angularity is its defining feature: the fragments have not traveled far enough to become rounded.
Most breccias are sedimentary, but the term is broad — there are also fault breccias (formed by crushing along faults), volcanic breccias, and impact breccias from meteorite strikes. The shared trait is broken, angular debris re-cemented into solid rock.
Breccia can be colorful and dramatic, and some varieties (like brecciated jasper or breccia marble) are prized as ornamental and lapidary stones.
Formation & geology
Sedimentary breccia forms where angular rock fragments accumulate close to their source, before water transport can round them. Typical settings include talus slopes and scree at the base of cliffs, alluvial fans, cave collapse deposits, and debris flows.
The sharp clasts are then buried and cemented by minerals such as silica, calcite, or iron oxide precipitating from groundwater.
Other breccias form by different processes: fault movement grinds and shatters rock into fault breccia; volcanic eruptions weld together angular ejecta; and meteorite impacts pulverize bedrock into impact breccia.
How to identify it
Look for angular, jagged fragments set in a finer matrix — the sharp edges are the key. If the clasts are rounded instead, it is a conglomerate, not a breccia.
Clast and cement composition vary widely, so hardness ranges accordingly; a calcite cement will fizz in acid while a silica cement will not.
Look-alikes: Conglomerate (rounded clasts), and brecciated mineral specimens (e.g. brecciated jasper) where a single mineral fractured and re-healed. Fault breccia is associated with shear zones and slickensides nearby.
Uses & significance
Decorative breccias are widely used as dimension stone, countertops, tiles, and facing stone — "breccia marble" is a classic ornamental material seen in classical architecture.
Lapidaries cut brecciated jasper, agate, and other gem breccias into cabochons and beads, valued for their bold, fractured patterns.
Geologically, breccias are important clues to past events: fault breccias map ancient faults, and impact breccias help identify meteorite craters. Crushed breccia also serves as construction aggregate and fill.
Frequently asked questions
What is the difference between breccia and conglomerate?
Breccia has angular, sharp-edged clasts that traveled little, while conglomerate has rounded clasts smoothed by water transport.
How is breccia formed?
Angular rock fragments pile up near their source (cliffs, faults, or impacts) and are then cemented together by minerals from groundwater.
Is breccia always sedimentary?
No. There are sedimentary, fault (tectonic), volcanic, and impact breccias. They all share angular fragments re-cemented into rock.
Is brecciated jasper the same as breccia?
It is related. Brecciated jasper is a single mineral that fractured and was re-cemented, while a true breccia mixes fragments of different rocks.
Breccia guides
In-depth guides for identifying, valuing, and understanding Breccia.
Breccia identified by the community
Real specimens identified with Rock Identifier.
