Identification guide · Minerals
How to Identify Franklinite
Franklinite forms black octahedral crystals or granular masses with a submetallic to metallic luster and a reddish-brown to black streak. Hardness runs 5.5 to 6.5, there is no distinct cleavage, and the mineral is weakly attracted to a strong magnet, unlike ordinary magnetite, which is more strongly magnetic. It is most reliably identified by its association with willemite and zincite in New Jersey specimens, since visually it can resemble magnetite or chromite from other localities.
Franklinite identification checklist
| Color | Black, Brownish black | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Metallic to submetallic | Turn it under a lamp: a true metallic shine reflects like polished metal and stays opaque at thin edges. |
| Transparency | Opaque | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 5.5–6.5 | It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. |
| Streak | Reddish brown to black | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | None (octahedral parting) | |
| Fracture | Uneven to conchoidal | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 5.1–5.2 | Very heavy, metallic heft (specific gravity about 5.15). |
| Crystal system | Isometric |
Extra tests for Franklinite
Magnet test. Weakly magnetic; a strong neodymium magnet will tug at it.
Franklinite look-alikes and how to tell them apart
Both sit near Mohs 5.5–6.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Franklinite vs Chromite →
Both sit near Mohs 5.5–6.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Franklinite vs Cassiterite →
Quartz is harder (Mohs 7) than Franklinite (Mohs 5.5–6.5), so Quartz will scratch Franklinite and not the other way round. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. Franklinite vs Quartz →
Both sit near Mohs 5.5–6.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
Franklinite identification FAQ
How can you tell if a rock is Franklinite?
Check the color (black and brownish black), the metallic to submetallic luster, a Mohs hardness of 5.5–6.5 and a reddish brown to black streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Franklinite?
Franklinite has a Mohs hardness of 5.5–6.5. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will.
What is the difference between Franklinite and Chromite?
Franklinite (Mohs 5.5–6.5, metallic to submetallic luster) and Chromite (Mohs 5.5, metallic to submetallic luster) look alike, but hardness, streak and heft usually separate them. See the full Franklinite vs Chromite comparison for a side-by-side table.
Is Franklinite worth anything?
Entry-level Franklinite specimens sell for around $2, typical collector pieces about $20, and premium examples $150 or more depending on size, quality and locality.
What color is Franklinite?
Franklinite is most often black and brownish black. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
