[Tomassci (OP)]

What is this guy?
Here's what I know:
Only small pieces are magnetic (i have only geomag magnets)
Black
My scissors won't damage this.

Best guess: magnetite.
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[mrsmith2211]

Could be slag.

[Bill S]

Assuming it’s not slag; and seeing it as a rock, rather than a mineral, it could be granitic.

Some more specific information would help.

Scissors are probably not the best tools for testing hardness; a good quality knife blade is better. Also, you will find suggestions on line for using other materials, e.g. glass, copper coin etc for testing.

It would be important to test different parts individually.  While testing hardness you can also scratch at least some of the minerals to find the colour of their streak.

If some of the dark spots are magnetic; your guess at magnetite could be right.
Just looking at a photo gives very limited information, but your specimen could include biotite mica and tourmaline.

Where did it come from?

[Tomassci (OP)]

I don't know. I found a rock, throwed it on earth and then i've seen it.

Where did it come from?

I don't know. I found a rock, throwed it on earth and then i've seen it. I guess it's propably from czech republic.

Streak is black (tested it on porcelain)

Hardness: comparable with quartz.
Something about 7.
Mictoimage:

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[Bill S]

OK; you’ve done some tests, that’s great.

I assume you got the black streak from a black mineral.  If I remember rightly, magnetite gives a black streak, so if the bit you tested is magnetic, you could be on to a winner.  As a cautionary note, though, I would say that porcelain tiles, especially the unglazed sides, are quite soft, so I would be surprised if you could obtain a streak from magnetite in that way.   

Hardness: comparable with quartz.
Something about 7.

That would be a fairly good estimate for the top of the hardness range for magnetite.

[Bass]

I agree, most likely magnetite if it's strongly magnetic. Otherwise it's probably ilmenite.