[Unreal]

Hello,

I have a question and maybe you can help me with this problem
I have a stone and i dont know what it is. Maybe it is an element, but maybe its an mineral.
I will give some info:
- it conducts electricity
- density is 2,4 g/cm3
- color is like silver (more grey), but sometimes it looks black, when i move the stone
- its very soft, but you cant scratch it
- when i put a piece of the stone into fire, nothing happens, when i put it in an acid and nothing happens (havent tried HCl yet)
Can you help me with this?

This is a piece of the stone:

[xiankai]

where did u obtain it from? that answer would help us alot.

[Dude]

I agree with Xiankai.  More detailed info is needed.  What you have looks like and has a density identical to elemental silicon.  However, I can guarantee that you did not find it in nature as a rock.  
1.  When you say it conducts electricity, do you mean that a voltmeter had very little resistance when the tips were applied or that a light bulb lit up when copper wires were linked between the rock and a battery?  Silicon is listed as a semi-conductor, so I don't think that you could establish it as a conductor without shaping the rock into more of a wire.

2.  If you used concentrated acid (ie H2SO4), you should be able to rule out Al and Ti, the only metals with that low of a density.  In addition, most metals would be expected to be hard.

3.  The only elements left would be some metalloids (Ge, As etc) and graphite (C).  Look up the densities in the Handbook of Chemistry and Physics and compare to what you listed (2.4 g/cm3).  If there is no match, then it is a "mineral".
 

[Unreal]

Thats the problem. I dont know were it comes from, i even dont know if it was found in nature.
Ill try to explain more...
Maybe 'it conducts electricity' is a bad conclusion. I hit it with an electrical flykiller thing and it gave lights, it does normally only with metals (and flies of course lol) and graphite, but im sure its not graphite.
And, when i used the metaldetector it detects it. But it can also the mineralparts in the stone...
Indeed, when i look to the density i see that silicon and boron can be the stone (im sure the density is around 2,3 or 2,4 g/cm3)...

Maybe its boron or silicium, or maybe silicion or boron with another material??

[Unreal]

i've asked it to a forum about minerals and crystals and they say its polycrystalline Silicon.

[Mitch]

Hit them against each other. Silicon will just sound a bit "different".

[Unreal]

It sounds very, how do you say that, light and high...

[Mitch]

Probably silicon then. When elemetns like Silicon strike each other they just don't sound metallic like and it also doesn't sound like to pieces of minerals striking each other either.

[P-man]

Maybe it's a mix??

[xiankai]

light and high --> high pitch?

soft = can scratch. i dont really get you, do u mean it can bend? or feels soft?

i doubt the silicon would be pure. there would most probably be impurities.

[hmx9123]

Before I ever read the post, I looked at the picture and immediately 'silicon' popped into my head.  I have a few pieces of silicon that were given to me as slag from a crystal puller at a wafer manufacturing plant, and they look exactly like this.  A lustry, hard surface that acts like a mirror, but is dark.  It's very jagged all around.  Try scratching things with it.  The outer layer should be silicon dioxide, so you should be able to scratch glass with it.  It should also be extremely sharp on the edges.  This is just anecdotal from looking at it, though.

[jdurg]

That looks VERY similar to my silicon sample in my collection, so I'd concurr with the Silicon guess.

[Unreal]

With soft i mean that i can hit easily pieces of the stone, but i can not make scratches on it with my nail (on graphite you can do that, but not on this).

And no, its not as pure as the silicon that is used for the electronic industry, that silicon is 99,99 - 99,999999% pure.
On a forum for minerals they told me this is silicon that is used for the steel industry with a purity of 97-99%
So its not very very pure.
And yes, the edges are very very sharp...

Anyway, they told me that this is (technical) polycrystalline silicon...what means that?

Here is the complete stone:

[jdurg]

Polycrystalline means that the lump is made of numerous different crystals and not one single crystal like the highly refined stuff is.  In a sense, all forms of silicon can be called crystalline as they all have some type of crystal structure.  Amorphous just has so many random crystals in there that there is no real differentiation between them, so overall it's called 'amorphous'.  Polycrystalline is a half-way inbetween form of amorphous and crystalline.  It has a distinct crystal structure to it, but there is more than one of those distinct structures in the composition so it's called 'polycrystalline'.