clast

Imagine smashing a rock with a hammer. The little pieces that fly off? Those are clasts! A clast is a fragment broken from a larger mass—most often rock, mineral, or hardened sediment. You can use ordinary words like fragment or shard in everyday English; clast is mainly the more precise, scientific label. Think of sand grains on a beach, pebbles in a streambed, or bits of shell embedded in sandstone.

The word comes from the Greek klastos, meaning "broken." Geologists use "clast" frequently when discussing sediments and sedimentary rocks—materials built from broken fragments of older rocks. You’ll also see it in related, more specific terms like bioclast (a shell or fossil fragment) or volcaniclastic (fragmented volcanic material). Outside earth science, the word is uncommon; people may use it metaphorically, but that’s more a stylistic choice than standard usage.

A clast is a fragment of rock, mineral, or other solid material that has broken off from a larger mass, typically through physical weathering, erosion, or impact. In many geological settings, clasts are then transported (by water, wind, ice, or gravity) and deposited elsewhere, but the core idea is simple: a clast is a broken piece of a larger whole. While it sounds technical, you encounter clasts every day. Each pebble in a pile of gravel is a clast; every grain of sand on a beach is a tiny clast.

The term is central to geology's clastic sedimentary rocks. These rocks, such as sandstone or conglomerate, form when accumulated clasts—sand, silt, and clay—are compacted and cemented together over long periods. Geologists classify clasts by size: gravel includes pebbles and cobbles, while silt- and clay-sized clasts can form fine-grained rocks like siltstone and shale. By studying the size, sorting, and roundness of clasts, scientists can infer how far the material traveled and the energy of the environment that carried it.

Beyond geology, clast sometimes appears in compound terms such as bioclast (a fragment of shell, bone, or fossil). People occasionally borrow it metaphorically, too, to evoke the idea of jagged, incomplete pieces—“clasts of memory,” for example—but that usage is intentionally literary and far less common than the geological one. Whether it is a massive boulder or a microscopic grain, a clast is a piece of a larger puzzle with its own unique history.