Article Nº 2668 Geology and mineralogy Vol. III · p. 22
Common opal
Original title: Opala comum
Common opal, hydrated silica, found in resinous and red (jasper opal) varieties on Porto Santo.
Hydrated silica. The resinous-looking, semi-translucent variety and the red variety (jasper opal) are found on the island of Porto Santo (‘holy harbour’).
Places mentioned