Idocrase. A mineral occurring in tetragonal crystals and also massive, of a brown to green color, rarely sulphur yellow or blue. It is a silicate of alumina and lime, with some iron and magnesia. It is common at Mt. Vesuvius.

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“Christ's ... yoke is an easy yoke; his burden like the burden of wings to a bird, that makes her fly the higher.”
–Richard Sibbes, The Bruised Reed and Smoking Flax