Gahnite

Gahnite is the zinc-bearing member of the spinel group, zinc aluminum oxide (ZnAl2O4). It typically forms well-shaped octahedral crystals that are dark blue-green, deep blue, green, or nearly black, with a vitreous luster.

Named after the Swedish chemist Johan Gottlieb Gahn, it is hard (Mohs 7.5-8) and durable like other spinels. The older name automolite is sometimes used for the blue-green variety, and intermediate zinc-magnesium types are called gahnospinel.

Though usually dark and opaque to translucent, gahnite is an interesting collector mineral and serves as an indicator mineral in the search for certain zinc and gem deposits.

Gahnite forms in granite pegmatites, in metamorphosed zinc-bearing ore deposits, and in high-grade metamorphic rocks such as schists, gneisses, and granulites. It is a notable indicator mineral near some massive sulfide (zinc) deposits.

It occurs with minerals such as quartz, feldspar, mica, garnet, sillimanite, sphalerite, and other spinels. Notable localities include Sweden (Falun), the United States (Massachusetts, Maine, Colorado), Brazil, Australia (Broken Hill), and India, often as octahedra embedded in matrix.

Look for hard, dark blue-green to black octahedral crystals with a vitreous luster and a grayish to brownish-white streak; it shows no cleavage and is quite tough (Mohs 7.5-8). Specific gravity is fairly high (around 4.4).

Its spinel-group octahedral form and hardness distinguish it from softer dark minerals. Look-alikes include common (magnesium) spinel, magnetite (which is magnetic and softer), and dark tourmaline or staurolite. Lack of magnetism rules out magnetite, and the octahedral habit and high hardness separate it from tourmaline and many other dark silicates.

Gahnite is rarely cut as a gem because most material is dark and only translucent, but rare cleaner blue-green stones are faceted for collectors. Its main practical importance is as an indicator mineral: its presence can signal nearby zinc sulfide ore bodies, making it useful in mineral exploration.

It has minor interest as a hard, durable spinel-group collector mineral and little metaphysical tradition. It is not a significant ore of zinc itself, since zinc is recovered mainly from sphalerite.