Obsidian. A kind of glass produced by volcanoes. It is usually of a black color and opaque, except in thin splinters.

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“If God give thee not only earth but heaven, that thou shouldest rule over sun, moon, and stars, and have rule over the highest of the sons of men, it would not be enough to satisfy thee, except thou hadst God himself.”
–Jeremiah Burroughs, The Rare Jewel of Christian Contentment