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A variety of datolite, usually having a botryoidal structure.

It occurs in nature also in boracite, datolite, tourmaline, and some other minerals.

A double salt of boric and silicic acids, as in the natural minerals tourmaline, datolite, etc.

The crystals were for a long time considered to be orthorhombic, and indeed they approach closely to this system in habit, interfacial angles and optical orientation; humboldtite was the name given by A. L�vy in 1823 to monoclinic crystals supposed to be distinct from datolite, but the two were afterwards proved to be identical.

Datolite is a mineral of secondary origin, and in its mode of occurrence it resembles the zeolites, being found with them in the amygdaloidal cavities of basic igneous rocks such as basalt; it is also found in gneiss and serpentine, and in metalliferous veins and in beds of iron ore.