Please identify this stone

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Offline arulamudhu

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Please identify this stone
« on: 30/09/2014 06:59:31 »
Hi, I am from the state of Tamilnadu, India. I found these stones in a heap of river sand at a construction site. The stone in image a & b show some crystalline structure inside. Please help me identify the stones. Is there a way to separate the crystal from the stone.


Offline Bill S

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Re: Please identify this stone
« Reply #1 on: 11/10/2014 19:39:23 »
Hi arulamudhu, It looks as though the experts are all waiting for someone else to make a start.  I know how frustrating it is to start a thread and get no response, so, although I have little chance of providing an identification, I will risk a comment or two in the hope that it sets the ball rolling. 

You identify a & b, so letís call the other two c & d, (from the top). 

Identifying specimens just from photos is inclined to be very difficult, especially for someone who is not familiar with the locality.  To a Cornishman like me c & d look like something granitic.  I would suspect the pinkish part might be feldspar, and the dark bits either mica or hornblende.  However, even to consider this as a wild guess it would be necessary to have a reasonable idea of the hardness, as well as the type of rock outcrop from which it might have come.

All four specimens appear weathered, almost certainly water-worn, but that does not necessarily mean they have travelled far from their origin.

Specimens a & b look very much like quartz, but, as you point out, they contain what look like crystals.  Once again, hardness is going to be a very important factor.  Quartz you would be unable to scratch even with a good quality knife blade, but you might be able to scratch it with a good steel file.  Feldspar is only a little easier to scratch than quartz.  If you have a piece of known quartz it would scratch feldspar, but it could be hard going.  Another common mineral that it could be is calcite, but that you would be able to scratch easily with a knife; also it would effervesce in acid. Dilute HCl is ideal. 

The crystals raise a distinct question mark.  I think you can safely rule out quartz crystals, but that does not necessarily rule out quartz.   The crystals could be pseudomorphs  after some other mineral that has somehow been removed and replaced by quartz.  The same could apply to feldspar, and even calcite.

Are you familiar with Mohís scale of hardness?  If not, I would suggest Googling it to see if you can sort out how hard your specimens are.  That would be a big step forward.

I hope this stirs someone with more expertise than I have to join the thread; even if only to point out how wrong I am.   [:D]     
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Offline Atomic-S

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Re: Please identify this stone
« Reply #2 on: 13/10/2014 04:08:52 »
The redder rock is a mystery to me. The whiter rock I strongly suspect to be primarily low-grade quartz, possibly of the chalcedony variety. The obviously crystalline part is another matter. Calcite? Hard to tell from the pictures.