Ignimbrite is a volcanic rock typically of rhyolitic composition. It is primarily composed of fine-grained igneous fragments with a medium to high percentage of volcanic glass. The overall colour is grey to bluish grey, but weathering can alter it to a pale pink due to iron oxidation. Ignimbrite is formed during violent eruptions, in a pyroclastic flow after it settled into layers of volcanic ash which fuse or 'weld' into a solid rock mass (modified from Wikipedia).
Ignimbrite used in masonry in the town of Orvieto, Umbria, Italy. Photo: J. Alean.Scenery eroded into ignimbrite layers, 'Fairy chimneys' in ignimbrite . Cappadocia, Turkey.