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Geology/rock identification

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Question
found rock
found rock  
Hi Joe,
I was hoping you could ID this rock. I found it in Martinez CA
It was very heavy for its size. I broke it open to look at the inside.
I also polished a section. It is slightly magnetic as in a magnet is attracted to
it but as far as it's magnetism is is fairly weak. It will affect a compass.
The exterior looks burnt and weathered. I'll attach a picture showing both exterior
and interior. Thanks for any help
Mike

Answer
My first GUESS is magnetite.  The greenish color kinda throws me off since magnetite is generally blackish gray with brownish tint in reflected sun, not green.  Whatever the rock is it has some iron in it; because of the magnetism.  

Magnetite is a mineral, one of the two common naturally occurring iron oxides (chemical formula Fe3O4) and a member of the spinel group. Magnetite is the most magnetic of all the naturally occurring minerals on Earth. Naturally magnetized pieces of magnetite, called lodestone, will attract small pieces of iron, and this was how ancient people first noticed the property of magnetism.

Small grains of magnetite occur in almost all igneous and metamorphic rocks. It is black or brownish-black with a metallic luster, has a Mohs hardness of about 6 (varying from 5.5 to 6.5)which means that a knife blade (5.5) will not scratch it. It has a black streak (meaning if you scratch it on a piece of porcelain the scratch will look black. It has a specific gravity of about 5.17 (when pure) so it will be heavy (water has a specific gravity of 1.0 and sandstone has a specific gravity of 2.65).  Sandstone the size of your specimen will be about half as heavy as your specimen and 5 X heavier than the same amount of water. I cannot tell, from the polished section, what the crystal structure is; however, magnetite's crystal structure is isometric - (secondarily) hexoctahedral with an octahedral crystal habit and generally fine granular to massive.  Looking at the polished section I believe that I can see some twinning which is characteristic of magnetite on the {Ill} side as both twin and composition planes, the spinel law, as contact twins.  The cleavage is indistinct, parting again on the {Ill}side, and is very good.

I hope this helps,

Joe

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Joe Norris

Expertise

I am an economic geologist. An economic geologist does mineral evaluations and appraisals of mineral or mining properties. I can tell you if your deposit has value - remember that a mineral deposit, no matter how good, only has value when mined. Any value assigned to a mineral deposit, in the ground, is only the speculative value that deposit.

Experience

I have been a economic geologist for most of my 35 year career. Although I have done work in perhaps 45 states and numerious countries much of my work has been in Appalachian coal, intermountain west gold and silver, and Arizona uranium.

Organizations
Past President of the Virginia Section of the American Institute of Professional Geologists and a certified geologist in twelve states.

Education/Credentials
BS Degree from Eastern Kentucky University. Work on MS Degree @ Eastern Kentucky University, Colorad School of Mines & Marshall University Numerious short courses on the value of mineral deposits and how to value same. Also several short courses dealing with the different types of geologic processes; sedimentary, igneous, metamorphic along with the mineral associated with each.

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