Referring to the one at the top of this thread: heavy and dark color suggest ultramafic. Doesn't seem macrocrystalline enough to be a granitoid. Heavy as well as the lack of porous texture suggest not volcanic extrusive, more likely intrusive or metamorphic. The greenish color looks olivine'ish. The black, possibly pyroxene? if it bothers an ordinary compass a lot, then there's a lot of magnetite in it. The mottling is the result of recementing of a breccia, suggesting metamorphism although this can happen in fault zones of materials of other types.
In one photo we see striations, which although associated with surface texture irregularity appear to be part of the rock structure. This would represent either a shear zone or depositional layers. In the context of this particular rock, it's a shear zone.
I vote for a recemented metabasalt breccia. If there are known greenstone belts in the vicinity, that'd pretty well nail it. If not, well, time for some other opinions (one coming up soon).
If you've got a compass handy, see if it deflects the compass more than just a tiny amount. If it does, as a minimum the rock is ultramafic and contains substantial magnetite. This looks like an easy rock to measure SpeeGee on if you know how, and that'll tell you roughly what the iron content of the rock is (although it won't tell you what minerals the iron is part of).
SECOND OPINION: This is a river cobble. The roundness of the thing indicates that apart from some small-scale texture, the rock is of uniform hardness and toughness and that it is well-cemented with material that's probably just about the same as the bulk of the brecciated material. And the surface is polished. There's only one good candidate, and that's quartz. The rock probably would have formed within a hydrothermal fault zone that included quartz intrusions cutting through mafic or ultramafic country rock, the quartz having been colored by the country rock and probably containing a lot of more complex silicates. The pitting would result from formation of crystals either softer or more chemically weatherable, that were insoluble in the quartz matrix. If the pits are square, the mineral in question was iron sulfide. If not square, the mineral was likely something else.
* * * * * Your turn, EU Citizen......... vad ni tanker?
