diatomaceous earth

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A light-colored porous rock composed of the shells of diatoms.

from the GNU version of the Collaborative International Dictionary of English.

• (Geol.) a deposit of fine, usually white, siliceous material, composed mainly of the shells of the microscopic plants called diatoms; also called infusorial earth, kieselguhr, and diatomite. It is used in polishing powder, and in the manufacture of dynamite.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A light soil consisting of siliceous diatom remains and often used as a filtering material or microabrasive.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a light soil consisting of siliceous diatom remains and often used as a filtering material