Porcelanite: Gemstone Information

Porcelanite is a name given to an ornamental rock found at Bucnik,Czech Republic. The material is an altered marly clay with characteristic sedimentary textures.Porphyries are types of igneous rocks which show comparatively large and well-formed crystals embedded in a groundmass of much finer texture. The porphyritic structure is considered to be due to a two-stage crystallization,or solidification, of the igneous magma at the time of its intrusion into the surrounding rock or extrusion on to the earth’s surface, when the magma contained already formed crystals enclosed in the molten liquid which subsequently solidified as a fine-grained groundmass. Porphyritic structure is common in rocks and it might be expected that a wide range of such a type of rock could well be used as an ornamental stone. This does not seem to be the case and the two porphyries first considered have more historical interest than as ornamental stones of modern application.The first of these is the green porphyry found in the province of Laconia in Greece, a rock with an olive-green groundmass with light green feldspar crystals sprinkled abundantly through it.
The green colour is due to included epidote and chlorite throughout the rock, which was known in classical times as ‘marmor lacedaemonium viride’. It was later known as ‘perfidio serpentino’, but the rock is not a serpentine. The quarries yielding this rock lie between the towns of Sparta and Marathonisi.The other and perhaps more important porphyry is the famous red porphyry of Egypt, ‘porfido rosso antico’, which was known in classical times as ‘porphyrites leptosephos’. This rock has a dark red groundmass,the colour being due to included piedmontite, a manganese mineral. In this groundmass is an abundance of small white and light pink feldspar crystals. The rock is quarried from a dyke some 25 m thick on the Jebel Dhokan mountain which lies some 40 km inland from the junction of the Red Sea with the Gulf of Suez and about 80 km eastwards from the Nile.It is doubtful whether the rock was known to the Egyptians but probably it was discovered in the reign of Claudius by the Romans who took it to Rome, where it was called ‘lapis porphyrites’ and later ‘The Stone of Rome’.
During the Roman occupation of Egypt, thousands of workmenwere employed in the quarries and the stone was transported to the Nileen route for the imperial city of Rome. A carved head of the Emperor Hadrian in Egyptian porphyry may be seen in the British Museum at Bloomsbury, London. The head is said to have been made in Egypt in about AD 130 and the carving was carried out by using copper tools fed with sand and emery.Llanoite or llanite is a reddish porphyry which contains mainly not only reddish microcline crystals in a dark brown matrix, but also small patches of quartz which show bluish gleams. This rock is found in Llano County, Texas, in the US.Although not strictly a porphyry, the rock called corsite, or sometimes napoleonite, might conveniently be mentioned here. It is a greyish rock containing lighter-coloured oval rings showing some radial structure.The material is an orbicular diorite or hornblende gabbro which occurs in the island of Corsica. There are two fine examples of this material in the form of vases in the Chateau de Malmaison outside Paris.