Porcellanite

Porcellanite is a white, by impurities also dark blue or gray, low metamorphic rock that emerges through silicification from the source rock. It forms under elevated pressure and temperature conditions in the border area of diagenesis from opal, resulting in sediments of the deep sea. The silica opal comes from the tiny skeletons of radiolarians, diatoms, sponge spicules or similar marine organisms with siliceous skeleton, rarely from volcanic incurred silica. The Opal Porzellanits is in the low-temperature modification of cristobalite and tridymite. Porcellanite goes in the course of diagenesis on in quartz, so that the rock is referred to as quartz - chert ( Chert ). The conversion of porcellanite rock in quartz horn is not only dependent on pressure and temperature, but also a function of time. In the North Atlantic, therefore, are the oldest known Porzellanite from the Lower Cretaceous.

Porzellanite also occur in lake sediments, where they may occur under suitable conditions of diatomaceous earth. They found, for example in the rocks of the fossil deposit Rott near Bonn.

Stone axes were made ​​from porcellanite in Ireland in the Neolithic period. As mining sites Tievebulliagh and Rathlin Iceland ( County Antrim ) are detected. 19 polished stone hatchets hid the " Malone Hoard " of Danesfort house.