NAGT Rock and Mineral Exchange - Desired http://nagt.org/nagt/teaching_resources/exchange/index.html Precambrian chert. Stromatolites. http://nagt.org/nagt/teaching_resources/exchange/postings/169280.html Tue, 03 Jan 2017 00:00:00 -0600 http://nagt.org/nagt/teaching_resources/exchange/postings/169280.html