This rock was found on a mountain on Peloponnesos, Greece.
It reacts to acid (vinegar) scratches glass but does not leave a streak.
I'm running out of options; though I'm still thinking it is a sort of limestone/dolomite.
How does it react to acid (vinegar)? Does it form small bubbles? If so its most likely a carbonate. Though carbonates are usually soft and glass is 5.5 - harder then most carbonates...
It does not make any bubbles (at least not visible) but you can hear a fizz.
I'm guessing it is something like breccia or an conglomerate.
If it was a carbonate, it would make bubbles (i.e. effervesce). What do you mean by "hear a fizz"? It could also be a Jasper if its hardness is closer to 7.
It looks like jasper.