A volcanic rock.
"The hikers carefully stepped over the jagged, black volcanic rocks scattered across the hardened lava field."
relating to or produced by or consisting of volcanoes
"volcanic steam"
"volcanic islands such as Iceland"
"a volcanic cone is a conical mountain or hill built up of material from volcanic eruptions"
igneous rock produced by eruption and solidified on or near the earth's surface; rhyolite or andesite or basalt
"volcanic rock includes the volcanic glass obsidian"
Of or pertaining to a volcano or volcanoes
"The volcanic ash from the recent eruption blanketed the entire valley, forcing residents to stay indoors for safety."