Definition of "obsidian"

The Neolithic culture from 8000 to 6000 B.C., however, was a brilliant period of the revival of crafts, the transformation of gathering into gardening, the growth of a cross-cultural obsidian trade, and the rise of towns.

1981, William Irwin Thompson, The Time Falling Bodies Take to Light: Mythology, Sexuality and the Origins of Culture, London: Rider/Hutchinson & Co., page 130

Looks like a yellow biscuit of a buttery cue ball moon / Rolling maverick across an obsidian sky

1975, “Nighthawk Postcards (From Easy Street)”, in Nighthawks at the Diner, performed by Tom Waits