Generally a fine crystalline or glassy igneous rock, which is formed by the solidification of molten magma or lava on the Earth's surface during volcanic action. Such as basalt, obsidian, etc.
See Alexandrite Tsarstone collectors guide, Definition of Volcanic rock, http://w.alexandrite.com/viewpage.html?id=GG-081-00004 (Generally a fine crystalline or glassy igneous rock, which is formed by the solidification of molten magma or lava on the Earth's surface during volcanic action. Such as basalt, obsidian, etc. ) (as of Apr. 06, 2026, 17:11 GMT).