The photos below show a clear-as-water, sap-like substance on a rock. This beautiful material is hyalite opal (a.k.a. glass opal) from Bohemia. Hyalite is also known as opal-AN, referring to amorphous opal having a network structure similar to silica glass, rather than opal consisting of silica gel spheres/colloids (most opal is this latter type). Hyalite opal is still hydrous silica (SiO2·nH2O). Under ultraviolet light (UV light; black light), this hyalite has a weak greenish glow.
Bohemian hyalite opal is famous and well known. It partially encrusts pieces of volcanic rock of Cenozoic age. The matrix is an alkaline mafic extrusive rock called vesicular porphyritic leucite tephrite, consisting of calcic plagioclase feldspar, leucite (a feldspathoid mineral), and augite clinopyroxene phenocrysts (= the blackish chunks in the gray rock). The whitish to cream-colored material near the edges of the hyalite mass is the rare mineral carbonate-rich fluorapatite (Ca5(PO4,CO3)3(F,O)).
Stratigraphy, Geologic Context & Age: upper Doupov Volcanic Complex (a.k.a. Doupovsské Hory Volcanics - the effusive phase from a cluster of shield volcanoes), Ohre Graben (a.k.a. Eger Graben - the northeastern branch of the European Cenozoic Rift System), Central European Alkaline Volcanic Suite, Circum-Mediterrancean Anorogenic Cenozoic Igneous Province, lowermost Oligocene to Lower Miocene.
Locality: hills of the Doupov Mountains north or northwest or west of the town of Valec (a.k.a. Waltsch), northwestern Bohemia/Czech Republic.
Hyalite opal (clear) (field of view ~4.6 cm across) on vesicular porphyritic leucite tephrite (upper Doupov Volcanic Complex, lowermost Oligocene to Lower Miocene) from the Doupov Mountains, northwestern Bohemia. Whitish = carbonate-rich fluorapatite.
Hyalite opal (clear) (field of view ~2.05 cm across) from the upper Doupov Volcanic Complex (lowermost Oligocene to Lower Miocene) in the Doupov Mountains, northwestern Bohemia.
Hyalite opal (clear) (field of view ~2.4 cm across) from the upper Doupov Volcanic Complex (lowermost Oligocene to Lower Miocene) in the Doupov Mountains, northwestern Bohemia.