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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“Even though I walk through the darkest valley, I will fear no evil, for you are with me; your rod and your staff, they comfort me.”
– Psalm 23:4