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I dug up some sand and small rocks in a stream bed near me and moved a powerful magnet through the wash picking up numerous small rocks and black sand.

Now, the black sand is definitely small crystalls of magnetite, but what about the small rocks which tend to be gray in color, some being brownish?

I thought pretty much anything you would be likely to encounter commonly (like in a stream bed) that is magnetic is magnetite. In other words, my understanding is that the only COMMON mineral that is magnetic is magnetite. But these small rocks do not appear to be magnetite crystals, and the brown ones definitely are not.

Does this mean there are magnetite crystals INSIDE the rocks? Like if I break them up will I find little tiny magnetite crystals within?

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Magnetite is indeed very magnetic. But other minerals are magnetic too, and given a strong enough magnet you will attract more than magnetite. Chromite is one that comes to mind that's also common in some black sands.

You might not always see the magnetite. Magnetite is mixed valence of iron 2 and 3 and as such it isn't stable in our oxygen rich atmosphere. Given enough time it will alter to a variety minerals such as hematite and iron hydroxides. So you might have magnetite in the core of a grain mantles by non magnetic hematite. It will look brown but it will still be attracted to a magnet.

Same thing with your grey rocks: they most likely have magnetite inside, hidden from view.

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There are many minerals that are magnetic. You used a relatively "strong" magnetic to extract them.

Does this mean there are magnetite crystals INSIDE the rocks? Like if I break them up will I find little tiny magnetite crystals within?

Perhaps. "Rocks" aren't homogeneous.

Hematite is relatively uncommon in my area, but if you're in northern Minnesota, then it is a common mineral. So what's common/uncommon depends where you're looking.

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  • $\begingroup$ None of the minerals on that page (except magnetite) are magnetic. My understanding is that magnetite is almost universally found everywhere, although in some areas only smaller particles are found. Why are you referencing hematite? Hematite is not magnetic. $\endgroup$ – Shaka Boom Nov 1 '15 at 2:05
  • $\begingroup$ I just used hematite as an example for a common vs uncommon mineral. $\endgroup$ – MaxW Nov 1 '15 at 2:09
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    $\begingroup$ Since you used a strong magnet to collect the rocks you have to consider "weakly" magnetic rocks. // So a "lodestone" will pick up a paper clip which is not magnetic itself. A paper clip doesn't have a magnetic field itself. The magnetic field in the paper clip is induced by the lodestone. So if paper clips were a mineral then they would be "weakly magnetic." $\endgroup$ – MaxW Nov 1 '15 at 2:14
  • $\begingroup$ Depending on particle size even paramagnetic particles may show visible attraction to magnets. Also, various iron oxides/hydroxides may form inhomogenous lumps of magnetic microcrystalls. Iron is everywhere. $\endgroup$ – Greg Dec 2 '15 at 0:58

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