The facts
What franklinite actually is
Mineralogically, franklinite is spinel group — ZnFe³⁺₂O₄, crystallising in the cubic system.
The colour is caused by fe³⁺ with Zn and Mn; opaque black with metallic to submetallic lustre. That is worth knowing because it determines what heating, light and time can and cannot do to the stone.
A refractometer gives nothing, because the material is opaque, and the specific gravity sits near 5.07–5.22; cleavage is none; octahedral parting.
Where franklinite comes from
The recognised sources are Franklin and Sterling Hill, Sussex County, New Jersey, USA.
One source only — which is unusual, and which means an origin claim naming anywhere else is either a mistake or a different material.
How franklinite is treated
Untreated. Prized mainly as part of the classic Franklin assemblage with fluorescent willemite and calcite; the fluorescence belongs to the associated minerals, not to franklinite itself.
None of this is a problem when it is said out loud. It becomes one the moment it is left out.
What franklinite is confused with
The usual confusions are Magnetite, Chromite and Hercynite.
Because the material is opaque, refractive index is no help. Specific gravity around 5.07–5.22, hardness of 5.5–6.5, streak and magnification do the work instead.
What franklinite is used for
Franklinite is bought by mineral collecting and metallurgy — and each of those trades wants a different form of the same material.
We quote each application in the unit that trade actually buys in, by the specimen.
Durability and care
Hardness 5.5–6.5; cleavage none; octahedral parting. That combination decides what it can safely be set into.
With no cleavage plane to worry about, the practical limit is abrasion rather than breakage, and it takes daily wear well.
Warm water, mild soap and a soft brush. Avoid ultrasonic cleaners on anything fracture-filled, stabilised, dyed or visibly included.
What franklinite costs
Indicative pricing for franklinite is Franklin assemblage specimens $25–$400.
That is a range, not a quote. Colour, size, clarity, cut and origin move the figure a long way inside it, and the only honest price is one given against a specific stone. Send us what you need and we will price it.
Questions
Is franklinite treated, and how?
Untreated. Prized mainly as part of the classic Franklin assemblage with fluorescent willemite and calcite; the fluorescence belongs to the associated minerals, not to franklinite itself.
Where does franklinite come from?
Franklin and Sterling Hill, Sussex County, New Jersey, USA. That is the only commercial source.
Is franklinite hard enough to wear every day?
Yes. At 5.5–6.5 on the Mohs scale with no cleavage plane, it handles daily wear well. The practical limit is gradual abrasion rather than chipping.
How much does franklinite cost?
Indicatively Franklin assemblage specimens $25–$400. Colour, size, clarity and origin move the price a long way within that, so treat it as a range and ask for a quote against a specific stone.
What is franklinite confused with?
Most often Magnetite, Chromite and Hercynite. Since the material is opaque, separation relies on specific gravity near 5.07–5.22, hardness and magnification.
Related stones
Looking for franklinite?
Tell us the size, the quality and what it is for. We source it, photograph it and quote it — with the locality named.
Ask about franklinite