Gahnite is a zinc-rich member of the spinel group, frequently identified by its distinct octahedral habit and high specific gravity. It is commonly found in metamorphic environments and zinc-bearing ore deposits, often appearing as dark, sub-metallic to vitreous crystals embedded in host rocks.
Is this gahnite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch gahnite with a known reference. Gahnite sits at Mohs 7.5-8, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: gray.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: bluish green, grayish green, deep leek green, greenish black, bluish black.
- 5Look at form & habitCrystal system: cubic. Typical habit: euhedral crystals, massive, striated.
Often confused with
Gahnite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Gahnite leaves gray, Spinel leaves white.
How to tell apart: Gahnite is noticeably harder (Mohs 7.5-8 vs. 5.5-6.5); streak differs, so Gahnite leaves gray, Magnetite leaves black; luster reads vitreous on Gahnite and metallic to submetallic, may be dull on Magnetite.
How to tell apart: Gahnite is noticeably harder (Mohs 7.5-8 vs. 5.5); streak differs, so Gahnite leaves gray, Chromite leaves brown; luster reads vitreous on Gahnite and metallic to submetallic on Chromite.
Often found alongside gahnite
Minerals reported to co-occur with gahnite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- ZnAl₂O₄
- Mohs hardness
- 7.5-8
- Density
- 4.38-4.60 g/cm³
- Colors
- Streak
- Gray
- Luster
- Vitreous
- Transparency
- Translucent to nearly opaque
- Crystal system
- Cubic
- Crystal habit
- Euhedral crystals, massive, striated
- Cleavage
- Parting {111}, indistinct
- Rarity
- Uncommon
- Uses
- Collector, Indicator mineral
- Host rock
- An accessory mineral in granites and granite pegmatites; in medium- to high-grade metamorphic rocks and metamorphosed base-metal sulfide deposits; in diasporites, formed by the low-grade metamorphism of bauxites
Where rockhounds find gahnite
Classic worldwide localities
- Franklin and Sterling Hill, Ogdensburg, Sussex Co., New Jersey, USA
- Mineral Hill and Patapsco mines, Carroll Co., Maryland, USA
- Deake and Chalk Mountain mines, Mitchell Co., North Carolina, USA
- Cotopaxi mine, Fremont Co., Colorado, USA
- Falun, Sweden
- Silberberg mine, near Bodenmais, Bavaria, Germany
Field-hunting tip
Where it forms: An accessory mineral in granites and granite pegmatites; in medium- to high-grade metamorphic rocks and metamorphosed base-metal sulfide deposits; in diasporites, formed by the low-grade metamorphism of bauxites. If you start seeing rhodonite, franklinite, calcite in float, you are in the right ground. Typical habit: euhedral crystals, massive, striated. Train your eye for that shape before scanning the outcrop.
Common questions
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Find gahnite on the map
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