The porphyry is a volcanic rock effusive: the most common on Earth's crust are ignimbrite riolitiche and riodacitiche. The porphyry petrograficamente, is a paste made of glass or microcrystalline background, which constitutes more than 65% are immersed in which small crystals (size 2 / 4 mm) as a percentage varies between 30/35%. The crystals are most abundant quartz, so that the rock is also given the name of "porphyry quarzifero". Considerably less is the presence of feldspar, small is that of micas. However Its color varies from light gray to a medium brown.