Re: What is the strength of naturally occurring magnets?

    I don't know the actual rock magnetization numbers.  I suspect that 
the maximum is pretty small.  Perhaps up to 4PiMr = 10 G for common 
rocks.  I know that people used small naturally occuring magnetite rocks 
as a compasses by floating them in water.  It is easy to find rocks that 
can be magnetized.  It is much harder to find one that has alrady been 
magnetized.  In order to magnetize a rock one has to have a strong 
magnetic field.  These exist only very rarely in nature.  I have heard 
about rocks being magnetized by being near the point where lightning 
struck the ground.  The field from the very high electrical current is 
certainly large enough to do the job.
     I suggest that you go a good technical library and look for a book 
on Rock Magnetization to get more data.  A quick look on Google turned up 
nothing useful.      

[note added by MadSci Admin:  I found several sites which have some good
information and some clues as to the strength of magnetite.  See

http://scitoys.com/scitoys/scitoys/magnets/magnets.html

and 

http://www.madehow.com/Volume-2/Magnet.html

The statement is made in the 2nd reference that NdFeB magnets can be
75 times stronger than magnetite.  If one looks at this site

http://www.forcefieldmagnets.com/catalog/index.php?cPath=23_37

one finds that their NdFeB magnets have a magnetic strength of about
12000 Gauss, which would imply that magnetite can be as strong as
160 Gauss, but I have no idea if that is typical of natural magnetite.
  - - John Link, MadSci Physics Admin
]