Franklinite is a distinctive black, metallic mineral primarily known for its occurrences in the unique ore bodies of Franklin and Sterling Hill, New Jersey. It is strongly magnetic and typically found as well-formed octahedral crystals embedded in a fluorescent matrix of calcite and willemite.
Is this franklinite?
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 franklinite with a known reference. Franklinite sits at Mohs ~6, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: reddish brown to black.
- 3Read the lusterHold the specimen under a strong light. Luster: metallic to semimetallic; may be dull.
- 4Match the color rangeCompare against the expected color range: black, brownish black.
- 5Look at form & habitCrystal system: cubic. Typical habit: crystalline, euhedral crystals, massive.
Often confused with
Franklinite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Franklinite leaves reddish brown to black, Magnetite leaves black.
How to tell apart: Streak differs, so Franklinite leaves reddish brown to black, Chromite leaves brown.
How to tell apart: Streak differs, so Franklinite leaves reddish brown to black, Hausmannite leaves dark reddish brown; luster reads metallic to semimetallic; may be dull on Franklinite and submetallic on Hausmannite.
Often found alongside franklinite
Minerals reported to co-occur with franklinite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- ZnFe₂O₄
- Mohs hardness
- ~6
- Density
- 5.05-5.22 g/cm³
- Colors
- Streak
- Reddish brown to black
- Luster
- Metallic to semimetallic; may be dull
- Transparency
- Opaque, translucent in fine fragments
- Crystal system
- Cubic
- Crystal habit
- Crystalline, euhedral crystals, massive
- Cleavage
- Parting on {111}, fair
- Rarity
- Uncommon
- Uses
- Collector, Ore of zinc
- Host rock
- In beds and veins formed by high-temperature metamorphism of Fe, Zn, Mn-rich marine carbonate sediments
Where rockhounds find franklinite
2 mapped spotsClassic worldwide localities
- Franklin and at Sterling Hill, Ogdensburg, Sussex Co., New Jersey, USA
- Found at Långban, Värmland, Sweden
- Occurs at Hraničná, Czech Republic
- Nayzatas, Dzhumart, and in the Ushkatyn deposits, Atasui area, Kazakhstan
- Pereval marble quarry, near Slyudyanka, Sayan Mountains, Siberia, Russia
- Koduru manganese mine, Srikakulam district, Andhra Pradesh, India
Field-hunting tip
Where it forms: In beds and veins formed by high-temperature metamorphism of Fe, Zn, Mn-rich marine carbonate sediments. If you start seeing zincite, willemite, calcite in float, you are in the right ground. Typical habit: crystalline, euhedral crystals, massive. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in New Jersey. Start trip planning there.
Common questions
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Find franklinite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
