Silver green granite


Among the silicate-based stone materials present in Italy, one in particular stands out for its colour and decorative features: Silver green granite, which a high-quality stone extracted at Argentera in the municipality of Settimo Vittone in the province of Turin.

The Silver reflections are produced by strips of a peculiar high-pressure ferriferous mica, called phengite. The splendid green colour, which is slightly opalescent when polished while delicate and varied in its natural form, is produced by the precious mineral jade, present in large crystals merging into the rock texture. Jade, which geologists call jadeite, is a particular silicate that forms at an extremely high pressure, and thus extremely deep into the Earth’s crust, in place of another very common mineral present in granite, that is, plagioclase. This means that, in ancient times, our Silver green granite was “ordinary” granite with milky white plagioclase, potassium feldspar, which is also white, translucid quartz and strips of black mica; then, about 90 million years ago, the rock mass got trapped by the converging the African and Eurasian plates. These two tectonic plates moved slowly towards one another, folding and crumbling at the edges, overlapping and heaping up in sheets (which geologists call nappes) on one another, thus creating the chain of the Alps, while our granite ended up at the bottom of the heap. Dragged tens of kilometres down into the bowels of the Earth, its original minerals could not withstand the weight of all the overlying rock and gradually collapsed to form crystals with a more dense lattice, including sodium pyroxene (jade). The lowest point in its trajectory was reached about 65 million years ago; from then on, the floating forces of our metagranite (granite transformed by pressure) began to prevail again and the rock mass rose “quickly” to the surface where it is found now.

This explains the unique nature of Silver Green Granite: very few natural rocks and, no commercial materials up to now, are hardened by such sharp changes in pressure. “Light” rock typical of the upper part of the Earth’s crust, our granite (with a density of about 2.7) plunged deep into the crust where the density of equilibrium is at least 3.2 (the value of basalt) transforming itself into a rock of the bowels of the Earth without suffering any damaging in bouncing back to the surface. This gives it unrivalled mechanical and aesthetic qualities.

History and Geography of the product

The Silver Green Granite quarry of Argentera is situated on the left side of the Dora Baltea valley, almost at the mouth of the valley in the Canavese plain, at an altitude of about 450 m a.s.l., about a hundred metres above the flood plain. Mining of the quarry dates back at least to the 18th century as is witnessed by the covering and wainscoting in the nearby parish church of Borgofranco; then, in the following century, the material was used in the structure of the tunnels on the Chivasso-Aosta railway line in the section between Ivrea and Pont-Saint-Martin. Other historical examples of its use include the Courthouse of Ivrea and other municipal buildings, churches, cemeteries, pavements and vine supports in the municipalities of Borgofranco, Tavagnasco and Settimo Vittone.

Since 1975, the quarry has been run by the Vuillermin Gualtiero company, which has adopted technologies suited to this particularly hard material, placing it at the top of its range of products.

Our current sale of Silver Green Granite has spread well beyond the local area, becoming established not only in the Alps (Aosta, Zermatt, Verbier, Crans-Montana, Innsbruck, Chamonix…) but also to more distant European cities (Berlin, Bonn, Budapest, Paris, Munich…) and even the other side of the ocean (Singapore Underground, the Presidential Palace of the Maldives, Hotel Plaza in Hong Kong…). Small pieces of material obtained from the scraps produced by the working of the blocks are transferred to the Stone Centre of Issogne where, with hydraulic shearing machines, they transformed into blocks with flat sides and a natural split. The blocks are then used as they are for dry walls, coverings and surfaces of traditional roads or gardens or are washed to produce ornaments, an application that inevitably leads to a comparison with its “sister” Birmanian jade …

> Technical data sheet of Silver Green Granite

Massive pilasters made of split Silver green granite cover the base of the façade of the parish church of Borgofranco. The work dates back to 1792.