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 5.5-6.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Franklinite leaves a reddish-brown streak.
- 3Read the lusterHold the specimen under a strong light. Franklinite typically shows a metallic luster.
- 4Match the color rangeCompare against the expected color range: black, brownish-black.
- 5Look at form & habitCrystal system: cubic. Typical habit: octahedral crystals, massive, granular.
Often confused with
Franklinite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Franklinite leaves reddish-brown, Magnetite leaves black.
How to tell apart: Streak differs — Franklinite leaves reddish-brown, Chromite leaves dark brown; luster reads metallic on Franklinite and submetallic on Chromite.
How to tell apart: Streak differs — Franklinite leaves reddish-brown, Hausmannite leaves brownish-red; luster reads metallic 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
- (Zn,Mn²⁺,Fe²⁺)(Fe³⁺,Mn³⁺)₂O₄
- Mohs hardness
- 5.5-6.5
- Density
- 5.0-5.3 g/cm³
- Colors
- Streak
- Reddish-brown
- Luster
- Metallic
- Transparency
- Opaque
- Crystal system
- Cubic
- Crystal habit
- Octahedral Crystals, Massive, Granular
- Cleavage
- None
- Rarity
- Uncommon
- Uses
- Collector, Ore of Zinc
- Host rock
- Metamorphosed Zinc Ore Deposits
- Typical price
- $10-100 per specimen
Where rockhounds find franklinite
2 mapped spotsClassic worldwide localities
- Franklin, New Jersey, USA
- Sterling Hill, New Jersey, USA
- Langban, Sweden
Field-hunting tip
Look in metamorphosed zinc ore deposits country — that is the host setting where franklinite typically forms. If you start seeing willemite, calcite, zincite in float, you are in the right ground. Field specimens usually show a octahedral crystals, massive, granular habit, so 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.
