Originally posted by Juston
Just found this outside my office. Pretty cool looking and really shiny, but kinda dull at the same time. Took the following pictures with my phone, but they really dont do it justice.
The rock itself feels really light.
Anyone know what it is?

It looks like Galena like someone else suggests, but that is a heavy mineral and typically cubical and much shinier and more grey and not black like you mention, but if its light it might be graphite, but to be sure we need you to rub it on some paper. Very cool looking rock, I would have picked it up too

Originally posted by mytheroy
I'm going to go with carbon, I found the same looking rock(s) while digging in a my dirt floor crawl space, really light, black dull and shiny

Yup...looks like a dirty piece of anthracite to me...coal.

Not knowing much about geology but am interested, can someone tell me why this would not be or could not be a black agate?

I hear they have a clearly discernible diffusion banding characteristic and am curious to know if this could be a charcteristic of this particular stone?

Originally posted by Juston
Just found this outside my office. Pretty cool looking and really shiny, but kinda dull at the same time. Took the following pictures with my phone, but they really dont do it justice.

The rock itself feels really light.


Anyone know what it is?

I propose this solution.


1)
Take a measuring cup fill it with enough water to cover the stone.

2)
Drop the stone into the water and see how much it has risen. The difference is the size of the stone.

3)
Weigh the stone on a kitchen scale. The weight divided by its size is it's density.


When we know its density, establishing its type should be a fairly straightforward process, and given that we also know how it looks, we can weed out the obvious incorrect candidates, so the measurements doesn't have to be super exact to be useful.


This ought to work, I think.

I have a minor in Geology, and it looks like coal to me. Flake off a small piece and crush it to dust, then see if it will burn. It is not obsidian if it is light, obsidian is volcanic glass and is relatively heavy.