The Gray Porphyroid, known as Branzi slate, is mined in the upper Val Brembana, on the border between Branzi and Carona, where outcrops belonging to the Collio formation have been quarried for centuries.
Its high resistance to loads, to decay and to the effects of frost have been known for a very long time, and the so-called “piode” (sheets for roofs) are an important item in the local economy.
Its geological origin dates back to about 280 million years ago in the Permian period, when the Lombard area was at a latitude near the Equator and was covered by a large shallow lake basin, where rivers carried and piled up abundant detrital deposits.
The first amphibians lived near these lakes and marshes, evolved from primitive fish.

After millions of years the sand and silt deposits became a very compact rock, consisting mainly of quartz (about 50%), which contains lava rocks and sediments accumulated over ten million years and reaching thicknesses of up to 1500 m.

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