Mineral Guide · Mineral

Gahnite

Gahnite is the zinc-rich spinel-group oxide, usually dark blue-green to black and associated with metamorphic rocks and pegmatites.

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Crystal structure

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Mineral properties

Formula: ZnAl₂O₄
Class: Oxides
Crystal system: Cubic
Typical colours: green, black, blue-green, dark blue
Lustre: Resinous, Vitreous
Hardness: 7.50-8.00
Specific gravity: 4.30-4.70
Crystal habit: Granular, Massive, Octahedral
Origin: metamorphic zinc deposits, granitic pegmatites, skarns and aluminium-rich rocks

What is Gahnite?

Gahnite is the zinc-rich spinel-group oxide, usually dark blue-green to black and associated with metamorphic rocks and pegmatites.

Mineralogy and structure

Reference formula: ZnAl₂O₄. Gahnite has the cubic spinel structure. Natural material commonly contains iron and manganese substitutions, so the locality and association are useful evidence. Gahnite belongs to the oxides and has a cubic crystal system. Typical hardness is 7.5–8 on the Mohs scale and specific gravity is approximately 4.3–4.7.

Colour and appearance

Typical colours include blue-green, green, dark blue, black. The usual octahedral, granular, massive habit and vitreous, resinous lustre are useful descriptive features, but colour alone is not diagnostic.

Formation and localities

Gahnite forms in or is associated with metamorphic zinc deposits, granitic pegmatites, skarns and aluminium-rich rocks. Important documented localities include the United States, Sweden, Norway, Brazil, Australia and Madagascar.

Collecting and care

Durable for display; preserve the matrix and avoid assigning the species from colour alone.

Sources and editorial review

Reviewed by NotOnlyRocks Editorial Team

Last reviewed: 31 August 2026

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