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Franklinite
Franklinite is oxide (spinel group, zinc), ZnFe₂O₄. In the hand, it rates 5.5 to 6.5 on the Mohs scale, and it runs iron black and brownish black. Black octahedral spinel from one famous New Jersey district, its companions give it away when the rock blazes green and red under UV light.
The geology.
Franklinite carries the formula ZnFe₂O₄. It belongs to the isometric system. It is classified as oxide (spinel group, zinc).
It sits at 5.5 to 6.5 on the Mohs scale. The surface reads metallic to submetallic. Light behaviour is opaque. Specific gravity runs 5.0 to 5.2. The streak is brown-black to reddish brown. It breaks with no cleavage; uneven fracture.
Black octahedral spinel from one famous New Jersey district, its companions give it away when the rock blazes green and red under UV light.
You will most often meet it as octahedral crystals in marble or granular ore with willemite and zincite.
The origins.
Essentially one place on Earth: Franklin and Sterling Hill, New Jersey (USA), the 'fluorescent mineral capital of the world'.
On value, the database notes: affordable ore; sharp octahedra in glowing matrix are collector pieces.
Traditional associations.
In energy work traditions, franklinite has little established lore; it stars in UV-mineral collections instead.
People buy on this language, so it belongs on the page. It belongs under the facts, though, not over them.
Spotting the real thing.
Black octahedral spinel from one famous New Jersey district, its companions give it away when the rock blazes green and red under UV light.
| Looks like | How to tell them apart |
|---|---|
| Magnetite | Magnetite is strongly magnetic; franklinite responds only weakly and streaks browner |
| Chromite | Chromite's streak is browner still, locality is the practical tell |
| Hematite | Hematite streaks red-brown and forms plates |
At-home tests.
You can run every one of these at a kitchen table. Good light matters more than gear when you are looking at franklinite.
| Test | What you should see |
|---|---|
| Magnet check | Weak magnetism (vs magnetite's snap) |
| By eye | Brown-black streak |
| UV test | companion willemite glows green, calcite red, franklinite stays dark |
Anything that scratches, burns or dissolves the piece is a lab job. Get it appraised rather than testing it apart.
Care & handling.
At 5.5 to 6.5 on the Mohs scale it handles careful wear but scratches against quartz and dust. Store it separately and take rings off before rough work.
It breaks with no cleavage; uneven fracture, which is worth knowing before you set it or drop it.
Warm water and a soft cloth clean almost everything. Ultrasonic tanks, steamers and household cleaners are where pieces get ruined.
A deeper look.
Extended detail on composition, sourcing, authentication and market value.
The physical record
The formula on record is ZnFe₂O₄. Crystal system: Isometric. Specific gravity: 5.0 to 5.2. Streak: Brown-black to reddish brown. Cleavage and fracture: No cleavage; uneven fracture.
How it reaches the market
Essentially one place on Earth: Franklin and Sterling Hill, New Jersey (USA), the 'fluorescent mineral capital of the world'.
What it gets confused with
Magnetite: Magnetite is strongly magnetic; franklinite responds only weakly and streaks browner. Chromite: Chromite's streak is browner still, locality is the practical tell. Hematite: Hematite streaks red-brown and forms plates.
What it is worth
Affordable ore; sharp octahedra in glowing matrix are collector pieces.
One more thing
Franklin's zinc ores fluoresce so famously that the town built a museum where whole walls of rock glow electric green and red.
An education-first guide.
No product links here. We do not sell franklinite, and a guide that only covers our own shelves would not be much of a guide.
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