Rock Crystal or Mountain Crystal. Any transparent crystal of quartz, particularly of limpid or colorless quartz.

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“God is a skilful physician. He knows what is best. God observes the several tempers of men, and knows what will work most effectually. Some are of a more sweet disposition, and are drawn by mercy: others are more rugged and knotty pieces: these God deals with in a more forcible way. Some things are kept in sugar, some in brine. God doth not deal alike with all, he hath trials for the strong, and cordials for the weak.”
–Thomas Watson, A Divine Cordial