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 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. Streak: brown.
- 3Read the lusterHold the specimen under a strong light. Luster: metallic, splendent to semimetallic, dull.
- 4Match the color rangeCompare against the expected color range: black, iron black.
- 5Look at form & habitCrystal system: cubic. Typical habit: disseminated, massive.
Often confused with
Jacobsite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Jacobsite leaves brown, Magnetite leaves black.
How to tell apart: Streak differs, so Jacobsite leaves brown, Franklinite leaves reddish brown to black.
How to tell apart: Streak differs, so Jacobsite leaves brown, Hausmannite leaves dark reddish brown; luster reads metallic, splendent to semimetallic, dull 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
- 5.5-6.5
- Density
- 4.76 g/cm³
- Colors
- Streak
- Brown
- Luster
- Metallic, splendent to semimetallic, dull
- Transparency
- Opaque, translucent on thin edges
- Crystal system
- Cubic
- Crystal habit
- Disseminated, massive
- Cleavage
- {111}, probably a parting
- Rarity
- Uncommon
- Uses
- Collector, Scientific research
- Host rock
- A primary mineral or an alteration product of other manganese-bearing minerals in some metamorphosed manganese deposits
Where rockhounds find jacobsite
Classic worldwide localities
- Bald Knob, near Sparta, Alleghany Co., North Carolina, USA
- Jakobsberg and Långban, Värmland, and Väster Silfberg, Dalarne, Sweden
- Sjö mine, near Grythyttan, Örebro, Sweden
- Benallt mine, near Rhiw, Lleyn Peninsula, Wales
- Kombat Cu–Pb–Ag mine, 49 km south of Tsumeb, and from the Otjiwarongo district, Namibia
- N’Chwaning mine, near Kuruman, Cape Province, South Africa, in crystals large for the species
Field-hunting tip
Where it forms: A primary mineral or an alteration product of other manganese-bearing minerals in some metamorphosed manganese deposits. If you start seeing hausmannite, galaxite, braunite in float, you are in the right ground. Typical habit: disseminated, massive. Train your eye for that shape before scanning the outcrop.
Common questions
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Find jacobsite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
