Hercynite is a member of the spinel group characterized by its deep black to dark green color and high hardness. It is typically found in high-grade metamorphic rocks resulting from the contact metamorphism of aluminous sediments. Collectors should look for its sharp octahedral habit, though it is frequently found as massive, irregular grains.
Is this hercynite?
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 hercynite with a known reference. Hercynite 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: dark grayish green to dark green.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: black.
- 5Look at form & habitCrystal system: cubic. Typical habit: euhedral crystals, granular, massive.
Often confused with
Hercynite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Hercynite is noticeably harder (Mohs 7.5-8 vs. 5.5-6.5); streak differs, so Hercynite leaves dark grayish green to dark green, Magnetite leaves black; luster reads vitreous on Hercynite and metallic to submetallic, may be dull on Magnetite.
How to tell apart: Hercynite is noticeably harder (Mohs 7.5-8 vs. 5.5); streak differs, so Hercynite leaves dark grayish green to dark green, Chromite leaves brown; luster reads vitreous on Hercynite and metallic to submetallic on Chromite.
How to tell apart: Streak differs, so Hercynite leaves dark grayish green to dark green, Spinel leaves white.
Often found alongside hercynite
Minerals reported to co-occur with hercynite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- FeAl₂O₄
- Mohs hardness
- 7.5-8
- Density
- 3.95 g/cm³
- Colors
- Streak
- Dark grayish green to dark green
- Luster
- Vitreous
- Transparency
- Opaque, translucent through thin edges
- Crystal system
- Cubic
- Crystal habit
- Euhedral crystals, granular, massive
- Cleavage
- Parting on {111}, probable
- Rarity
- Uncommon
- Uses
- Collector, Scientific research
- Host rock
- In high-grade metamorphosed ferruginous argillaceous sediments and in some mafic and ultramafic igneous rocks
Where rockhounds find hercynite
Classic worldwide localities
- Peekskill, Westchester Co., New York, USA
- Whittles, Pittsylvania Co., Virginia, USA
- Rice, near San Carlos, Gila Co, Arizona, USA
- Nacetm and Hoslau, near Ronsberg, Czech Republic
- Schenkenzell, Black Forest, Germany
- Pl6ssberg, Bodenmais, Bavaria, Germany
Field-hunting tip
Where it forms: In high-grade metamorphosed ferruginous argillaceous sediments and in some mafic and ultramafic igneous rocks. If you start seeing magnetite, corundum, ilmenite in float, you are in the right ground. Typical habit: euhedral crystals, granular, massive. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify hercynite?+
Where is hercynite found?+
What rocks look like hercynite?+
What minerals are found with hercynite?+
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What is hercynite used for?+
Find hercynite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
