Geology/what are these
Can you explain me what are these (the length is about a cm) on the pictures, I send you?
ANSWER: Hi Kolin,
It looks like you have a couple of pieces of "quartzite". That's a very hard, metamorphic rock that was changed, through the actions of pressure and temperature, from pure sandstone. It can be pretty much any color from white to black. One of the ways you can tell it's quartzite is how smooth it is (all the crystal grains can melt together) and the fracture pattern on the brown specimen. It looks much like fractured glass.
Here's a site that has a couple specimens that resemble yours:
Hope this helps.
[an error occurred while processing this directive]---------- FOLLOW-UP ----------
Thank you a lot but when I see the structure of my minerals I find them different. Obviously the pictures are not good. I wrote you because I can't find similar minerals in Internet and I spent many days to look for. I'm sending you one picture more.
Thanks for sending a follow-up to your question. This new picture and a review of the other two got me to thinking about another possibility. Chert and Flint are cryptocrystalline minerals that look very glassy. In my experience they are not normally brown but they can be.
Can you scratch the surface with a pin or knife blade. Chert and flint are harder than steel so you should not be able to easily scratch them. Here's a couple sites that give an idea what they look like;
flint - http://commons.wikimedia.org/wiki/File:Flint.jpg
brown chert - http://www.texasbeyondhistory.net/trans-p/nature/images/cienega-brown.html
A sedimentary rock called "mudstone" is very finely crystalline and can look like your specimens. It is softer and will scratch with a pin or knife point.
The only other cryptocrystalline mineral it might be would be obsidian but they don't look like that to me.
It's very difficult to identify a specimen like yours from a photo. Your best bet is to take them to a local expert, the geology department of your local university, members of a local rock club, anyone that can actually hold the rock.
Hope this helps.