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Breccia is very similar to conglomerate. The main thing that is different is that the pebbles in breccia have not been rounded by the action of moving water as in a conglomerate.
Breccia is made up of angular pebbles cemented together. The size of these rock fragments is over 2 mm in diameter.
After the pebbles have been deposited they are compacted by the sediments that pile up on top of them. Over very long periods of time the pebbles become cemented together by minerals.
Silica, calcite, and iron oxides are the most common cementing minerals. The angular pebbles in the breccia are easily visible to the naked eye.