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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“Every comfort that the saints have in this world is a promise to them of those eternal mercies which the Lord hath provided for them: as every affliction that the wicked have here, is but the beginning of sorrows.”
–Jeremiah Burroughs, The Rare Jewel of Christian Contentment