Jacobsite is a manganese-rich member of the spinel group, typically identified by its dark, often metallic appearance and distinct magnetic properties. It is predominantly found in metamorphic manganese deposits where it occurs as octahedral crystals or dense masses. Collectors often look for it in classic localities like the Franklin mining district or Swedish manganese mines.
Is this jacobsite?
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 jacobsite with a known reference. Jacobsite 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. Jacobsite leaves a black streak.
- 3Read the lusterHold the specimen under a strong light. Jacobsite typically shows a metallic to submetallic luster.
- 4Match the color rangeCompare against the expected color range: black, dark brownish-black.
- 5Look at form & habitCrystal system: cubic. Typical habit: octahedral crystals, massive, granular.
Often confused with
Jacobsite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Luster reads metallic to submetallic on Jacobsite and metallic on Magnetite.
How to tell apart: Streak differs — Jacobsite leaves black, Franklinite leaves reddish-brown; luster reads metallic to submetallic on Jacobsite and metallic on Franklinite.
How to tell apart: Streak differs — Jacobsite leaves black, Hausmannite leaves brownish-red; luster reads metallic to submetallic on Jacobsite and submetallic on Hausmannite.
Often found alongside jacobsite
Minerals reported to co-occur with jacobsite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Mn²⁺Fe³⁺₂O₄
- Mohs hardness
- 6
- Density
- 4.7-4.9 g/cm³
- Colors
- Streak
- Black
- Luster
- Metallic to Submetallic
- Transparency
- Opaque
- Crystal system
- Cubic
- Crystal habit
- Octahedral Crystals, Massive, Granular
- Cleavage
- None
- Rarity
- Uncommon
- Uses
- Collector, Scientific Research
- Host rock
- Metamorphosed Manganese Ore Deposits
- Typical price
- $10-100 per specimen
Where rockhounds find jacobsite
Classic worldwide localities
- Jakobsberg, Sweden
- Franklin, New Jersey, USA
- Langban, Sweden
- Kalahari Manganese Field, South Africa
Field-hunting tip
Look in metamorphosed manganese ore deposits country — that is the host setting where jacobsite typically forms. If you start seeing hausmannite, magnetite, bixbyite 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.
Common questions
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Find jacobsite on the map
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