Phonolite

Phonolite, or Clinkstone, a compact, grey, semi-crystalline lava, consisting mainly of sanidine felspar and nepheline, and named (from the Greek phone, "sound") on account of the ringing sound it emits when struck with a hammer. The rock usually contains some hornblende and nosite, and is sometimes porphyritic. Its specific gravity is about 258. It is of Tertiary age, and occurs among the lavas of Auvergne and Bohemia. In thin slabs it is used for roofing.