Tripoli
ahd-5
noun.
A porous, lightweight, siliceous sedimentary rock composed of the shells of diatoms or radiolarians or of finely weathered chert, used as an abrasive and a polish.
Wiktionary, Creative Commons Attribution/Share-Alike License
noun.
A
sedimentary
rock
composed of the shells of
diatoms
etc
Word Usage
"This kind of tripoli has been lately dif - covered in Scotland."