Porphyry is available in different colours. Nevertheless, the purple one is mostly known and historically raised it to its actual prestige. During Pharaonic Egypt times, it would seem that it was unused and unknown. That said, other similar stones were used in Predynastic Egypt for making vases, but it would seem that this ceased afterwards. However, it was discovered under the Ptolemies. At that time, Egypt had the only known deposit of Antiquity, located near the Jebel Dokhan in the middle of the desert. This stone was considerably used and exploited during Ancient Rome, knowing the exorbitant price of the labour required to extract it and transport it to the cities. The bright red porphyry was very popular, as it referred to the colour of power.
It is known for its outstanding hardness, which even dethroned that of calcareous marble, and consequently required years of work and carving, given the tools used at that time. It was therefore considered the most prestigious stone of the Ancient Rome Era, an undeniable symbol of imperial power, strictly reserved for the ornamentation of monuments. From the Middle Ages onwards, its exploitation was suspended and the craftsmanship that goes into producing it was lost. As a result, the 18th century is known to have reused porphyry stone from ancient Roman monuments for its architecture. Shortly afterwards, new sites appeared in Russia and Sweden. Undoubtedly, the symbolic and imperial part of this stone has never ceased to stir the imagination since the deposits from which it originated vanished, even inventing tenacious preconceptions such as the one that makes us believe that Napoleon I’s tomb is made of this material whereas it is quartzite.
All because this stone was very difficult to work with, and because its purple hue was often associated with the blood of Christ, which further reinforced its mythical and legendary aspect. Decorations carved from this magmatic mineral were found in the Middle Ages, and Vasari started a legend that the secret of carving was found in Florence in the 16th century. In truth, those found until the end of the 18th century were reworked ancient pieces. Either porphyry was mixed with marble and bronze to recreate new objects, or they were simply reshaped to create new pieces. This makes it very difficult to accurately date discovered objects. What remains certain is that in the Seicento, in Rome, a lot of pieces, including vases, were exported all over Europe until the complete decline of this industry at the end of the 18th century. Moreover, the discovery of sites in Sweden and Europe led to its gradual banalization, limiting its use only to the carving of precious objects known as «eternal ».
To date, the main quarries are located in Trentino, in Italy (1.5 million tons per year), in Australia, Mexico and Argentina, but also in Belgium with a production of 1.8 million tons per year in the Ouenast quarry. Additionally, Corsica pink porphyry, from Algajola, is very popular. This highly resistant mineral has been used in the construction of several famous structures such as the Delta Plan in the Netherlands or the Channel Tunnel. It is also used for the railways of the TGV lines and for asphalt roads, or for paving stones.
Porphyry comes from ancient Greek πορφύρα, porphýra meaning purple, the most known variety of this time.
In petrography, it refers to any igneous rock characterized by large feldspar crystals embedded in an aphanitic paste, which makes it belong to the andesite group. A texture known as « porphyric » indicates the presence of feldspar phenocrysts larger than the surrounding crystals. Since Antiquity, certain varieties have been used for the ornamentation and sculpture of columns, wall plates and basins. However, two varieties stand out from the others, as they were widely used in ancient Rome, and are classified as ancient marbles :
1- Red porphyry or lapis porphyrites is an andesite with paleovolcanic facies coloured by pink epidote called piemontite, a mineral belonging to the silicate group. The quarries from which it is extracted are located in the mountainous range of the Egyptian eastern desert, west of Hurghada on the Jebel Doukhan. The latter was formerly known as Mons Porphyrites, or Mons Igneus. This variety was mainly exploited during the Roman Era, which made it a noble and imperial material, mainly used for the construction of imposing buildings, majestic sarcophagi and other monumental decorations.
2- Green porphyry or lapis lacedaemonius found in Lacedemona, ancient Sparta, is an andesite with aphanitic paste and paleovolcanic facies, made of large labradorite crystals transformed by pistachio epidote. It also contains rare black pyroxenes. It is mined mainly in the Greek Peloponnese. It was already used during the Minoan and Mycenaean periods, and was widely distributed in Rome under the Flavii. It was also much sought after during the Middle Ages and the Renaissance. As for its use, it was intended for sculpture, as it was only available in small blocks.
Porphyry stone has lost the nobility that once made it a precious material. Nowadays, it is mainly used to make paving stones, because it is very solid, or for decoration, if it is skilfully polished. But it has also become famous because of its gold content. Two examples of regions where it is exploited remain interesting :
At the end of the Ordovician, the second geological system, and at the very beginning of the Silurian, geology shows us that a volcanic arc was formed from the Brabant massif to Flanders. These intrusions are probably posterior to the obstruction of the Tornquist Ocean and the collision between Avalonia and Baltica. They may have been caused by the subduction of an oceanic crust between two blocks of Avalonia, as volcanism is explosive there. The two main emission centres are located in Lessines and Quenast, south of Brussels. Indeed, a 433-Ma hornblende quartz-diorite forms the 2 km diameter vertical subcircular intrusion at Quenast. This calc-alkaline volcanism is found in the Caledonian Condroz Strip, as well as in the Llandovery, in the eastern region of Brabant. Additionally, various Bouguer anomalies have been discovered through geophysics, and the link between them and the intrusive andesites remains an unsolved mystery to this day. Some scientists believe that a Precambrian crustal block of granitic nature is responsible for the gravity anomaly. They justify their assumptions by the existence of a genetic incompatibility between a granitic parent magma and the intrusive rocks of Quenast. Finally, the western part of the Brabant massif is highly deformed because of this structure.
Time for gold hunting! The project is located in a gold sector that includes the Tres Cruces, Shahuindo, Alto Chicama, and Comarsa mines. Geologically, the Arena deposit is composed of sandstone, quartzite and thick sedimentary sequences dating back to the Jurassic and Cretaceous eras. They are intersected by a silica-rich layer: a subvolcanic dacitic porphyry. Two different categories of mineralisation have been identified on the property: a gold breccia zone and a copper-gold porphyry. In addition, the Arena hosts resources of up to 23.8 million tons at a grade of 0.7 g Au/t. This represents 536,300 ounces of gold valued at $400 per ounce on the long-term market.
Moreover, it was not only used in Europe. The Aztecs used it to make pieces, such as the famous circle of the Sun, composed of eight concentric circles with religious significance. Representation of the sun, symbolization of the four elements, illustration of the eighteen months of the calendar year, or figuration of the twenty days that made up the month, are emblematic expressions of this Aztec circle. Finally, this magma has not yet revealed all its secrets.