Clear glass. This is attributed to Grant Featherston, Australian Designer. His buttons are backmarked Darian with G.F.D. (Grant Featherson Designs). One of the best online references: http://www.ausbuttonhistory.com/?page_id=1788 or https://en.wikipedia.org/wiki/Grant_Featherston
The above is based on the National Button Society's Classification System.
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