Spongolite

Spongolite is a sediment or sedimentary rock composed principally of the remains of largely articulated fossils of rigid-bodied siliceous sponges.

In contrast, spiculites are sediments and sedimentary rocks composed of mostly disarticulated, discrete siliceous sponge spicules.

[2] In Brazil, the term spongolite (also either spongillite or spongiolite) is defined differently as a nonmarine, freshwater, lacustrine sediment "..essentially composed of siliceous sponge spicules of the Demospongiae Class, associated with clay, sand, organic matter and diatom residues."

The siliceous spicules of the fossil sponges comprising spongolite makes the material hazardous to handle by being highly abrasive.

Because the spicules are embedded in soft, opaline matrix, the abrasion damage is not as immediately apparent as it would be from sandpaper or rough bricks.