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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“Whatever we find lovely in a friend, or in a saint, ought to elevate our affections: we should conclude that if there is so much sweetness in a drop; there must be infinitely more in the fountain. If there is so much splendour in a ray, what must the sun be in its glory!”
–Henry Scougal, The Life of God in the Soul of Man