Horton,You are correct. Most magnetic m...

Horton,
You are correct. Most magnetic minerals are dark or metalic in color. There are many iron, nickel, and magnesium minerals that are magnetic and form needle-like crystals. I'd have to check my mineral books to give you further details.

It would help to know just HOW magnetic the substrate is....highly magnetic or does it have a lesse magnetic attraction? If we knew the strength of the magnet we would be able to narrow down the potential minerals that are stuck to it.

My experience with FECO3 is limited to sedimentary iron carbonates which are slightly magnetic and deposited in conjunction with biological activity.
Other sedimentary deposited hematite on Earth is much more magnetic.

Biologic and non-biologically (metamorphic/igneous) deposited magnetite is much more magnetic than hematite, siderite, and altered iron pyrites (and other upper crust minerals).

Iron containing meteorites can also have highly magnetic crystals. Pieces of the octahedrite meteorite crystals could weather to form a needle shaped magnetic crystal like this.

Eroded Meteorite shard? magnetic mineral needle crystal? Fracture fill magnetic material? Biological formed magnetic fossil?mineral? Or something else...
The light color could represent oxidation/corrosion on a darker or metallic substrate.

What is the scale here...anyone?