Mineral Guide · Mineral

Howlite

Howlite is a soft white calcium borosilicate, usually found as nodules with grey web-like markings and widely used as a decorative stone.

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Crystal structure

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Mineral properties

Formula: Ca₂B₅SiO₉(OH)₅
Class: Borates
Crystal system: Monoclinic
Typical colours: colourless, white, grey-white
Lustre: Pearly, Vitreous
Hardness: 3.00-3.50
Specific gravity: 2.50-2.60
Crystal habit: Botryoidal, Massive, Nodular
Origin: evaporite-associated borate deposits and arid sedimentary environments

What is Howlite?

Howlite is a soft white calcium borosilicate, usually found as nodules with grey web-like markings and widely used as a decorative stone.

Mineralogy and structure

Reference formula: Ca₂B₅SiO₉(OH)₅. Howlite is monoclinic and commonly fine-grained or nodular rather than well crystallised. Dyed howlite and unrelated white materials are common in the trade, so natural colour should be stated carefully. Howlite belongs to the borates and has a monoclinic crystal system. Typical hardness is 3–3.5 on the Mohs scale and specific gravity is approximately 2.5–2.6.

Colour and appearance

Typical colours include white, colourless, grey-white. The usual nodular, massive, botryoidal habit and vitreous, pearly lustre are useful descriptive features, but colour alone is not diagnostic.

Formation and localities

Howlite forms in or is associated with evaporite-associated borate deposits and arid sedimentary environments. Important documented localities include Canada, the United States, Mexico, Turkey and South Africa.

Collecting and care

Soft and porous; protect from scratches, dyes, oils and prolonged water exposure.

Sources and editorial review

Reviewed by NotOnlyRocks Editorial Team

Last reviewed: 31 August 2026

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