Porcelanite from the Czech Republic, also called Czech Snow Quartz.

Porcelanite is a white, due to impurities also dark blue or gray, low metamorphic rock, which is created by silicification from the source rock. It forms under elevated pressure and temperature conditions on the border of diagenesis of opal, which forms in sediments of the deep sea. Opal silicon comes from the small skeletons of radiolarians, diatoms, sponge needles or similar marine life with a silicon-like skeleton, more rarely from volcanic silica. The porcelain opal is present in low temperature modifications of cristobalite and tridymite. In the course of diagenesis it goes from porcelain to quartz, so that the rock is then called quartz-chert (chert). The transformation of porcelain into quartz is not only dependent on pressure and temperature, but also a function of time. In the North Atlantic you can therefore find the oldest known porcelain from under chalk.