''Chiton olivaceus'' can reach a length of {{convert|32|-|40|mm}} and a width of about {{convert|16|mm}}. These large chitons have carinate plates with strongsstrong ribs. The shell is oblong and oval. In the front and rear plates ribs have a radial pattern. The intermediate [[Valve (mollusc)#Chitons|valves]] show a sharp beak and rounded sutural plates. Colors are very variable, ranging from olive-gray (hence the common name) to yellow-brown, sometimes black, orange, red or yellow. The girdle surrounding all of the valves is quite large and covered by bristles and scales.<ref name=BB>[http://www.mondomarino.net/ricerca/index.asp?p=9&cat=20&view=dett Mondo Marino] {{webarchive|url=https://web.archive.org/web/20160304092325/http://www.mondomarino.net/ricerca/index.asp?p=9&cat=20&view=dett |date=2016-03-04 }}</ref><ref>{{Cite web |url=http://www.portofinoamp.it/it/schede-organismi/457-chiton-olivaceus-chitone-oliva.html |title=Area Marina di Portofino |access-date=2015-09-03 |archive-url=https://web.archive.org/web/20150924080153/http://www.portofinoamp.it/it/schede-organismi/457-chiton-olivaceus-chitone-oliva.html |archive-date=2015-09-24 |url-status=dead }}</ref><ref>[http://wildlife-archipelago.gr/wordpress/marine-inverts/green-chiton-chiton-olivaceus/ Wildlife Archipelago]{{dead link|date=August 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>
The teeth of these grazers of algae <ref>D. I. MacKinnon,Daphne E. Lee,J. D. Campbell [https://books.google.com/books?id=Bqpb_rmUzo0C&dq=chiton+olivaceus+grazer&pg=PA261 Brachiopods Through Time]</ref> are composed of magnetite, the hardest material usable by a living being.