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Geology/Can you help?


The rock The stone
The rock The stone  
I found this rock a few years ago. Forgot I had it till I happened to stumble across it the other day. My question is what sort of rock is it? I found this rock in the northern part of the Michigan mitten. Atlanta Michigan to be precise. I have pictures attached to this. They came out much darker then the rock really is. It is a harder stone with a shiny reflective property. Its a smaller sample that I found maybe the size of a quarter. still its light weight for its size. It looks to have rougher surface yet when you hold it it feel very smooth in most spots except where it looks like it may have fragmented or fractured off. I held a lighter up to it for several minutes. When I took the flame away I could still clasp my hand and it only felt slightly warm where the flame was held to and not much change if any at all. very un conductive. I did bite down on it. Probably horrible for my teeth but it was in the name of information. it seemed grainy and like it was going to break or flake. Then no matter how hard I bit nothing happened. I hit it somewhat hard with a hammer it still did not fracture at all. its coloring is slighting darker then that of the dime in the picture. I have looked for the rock on the internet the last few days I found similar rocks but nothing that really matched. Again my question is can you help identify this rock or what it could be?

I will give it my best shot; but I can tell you that it sure looks like coal.  Except for trying to get the sample to burn and the hardness I would say that it is coal.  Coal will not burn with a lighter because the lighter gets too hot before the coal starts but it should smell.  Coal, depending on type, varies significantly in hardness from 2.5 to 4 with anthracite being at the upper end; and this does look like anthracite.  This is also about the size of transported coal.  

1) I need to know the hardness of the rock.  Hardness of a rock is based on what mineral/rock type will scratch another mineral/rock type.  Since you don't have ten different minerals, that make up that scale, you can use some artificial harnesses from common things.  The scale goes from 1 to 10 with talc = 1 and diamond = 10, but you can use common things to determine hardness.  Your fingernail =~ 2; penny =~ 3; knife blade = 5.5.  Scratch your rock and tell me what its hardness is.  It will probably be < 5,  but let me know if a knife blade scratches it.

2) I need to know the streak of the rock.  The streak is the color of the rock when the rock is scratched by porcelain.  If you can find a piece of porcelain and scratch the rock on the porcelain the color that shows up on the porcelain is the streak.  If you have or can find a piece of porcelain, fine, but if you can't you might want to get what is called a streak plate; they are in-expensive and particularly if you like rocks in-valuable in determining rock types.  If the rock is coal it will have a gray or black streak.

3) The last thing that I need to know and you may or may not be able to do this - is the specific gravity.  The specific gravity of a rock is its weight relative to volume.  The standard is water which has a specific gravity of one.  The way that you determine the specific gravity, without a jolly balance (a machine used for this purpose), is somewhat complicated.  You need to find a beaker (graduated in metric) large enough to contain the rock.  Fill the beaker with enough water to cover the rock.  Take a measurement of the water level.  Add the rock and take another measurement of the water level.  Subtract one from the other; which gives you the quantity of water displaced.  Determine the weight (or mass) of the rock.  Divide the weight by the water displaced.  This will give you the Specific Gravity.

Let me know you answers.


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Joe Norris


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.


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.

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

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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