7
$\begingroup$

What is this black rock? I found it beneath the waters of Lake Ontario near the shoreline, it is a dark greyish-black with small flecks of mineral deposits on it that are visible in the light. The acid test resulted in NO fizzing, and it is non-magnetic. Tried chipping it off, wondering if it was painted, and it remained black, there are fine streaks on it as well that you might be able to see on the photo. Tried drilling a hole in it to see for certain if it was black and was BARELY able to make a dent, a hammer only scratched the surface so it is very durable. Hardness level is a 6-7, and it left a grey streak on paper.

Luster is obviously non-metallic, it is very smooth but that might be because from the water hitting it where I found it. It does have some imperfections all around it but you can't feel it, it is also on the heavier side.

enter image description here

enter image description here

$\endgroup$

closed as off-topic by Leukocyte, Gimelist, uhoh, Fred, Semidiurnal Simon Aug 27 at 19:17

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions about rock identification requests are off-topic. For more information, see the announcement on meta." – Leukocyte, Gimelist, uhoh, Fred, Semidiurnal Simon
If this question can be reworded to fit the rules in the help center, please edit the question.

  • 2
    $\begingroup$ Easier to identify the oak table than the black rock! It could be a dark-weathering sandstone, but my guess is that it is basalt, with a few late-stage back-veins. It would be helpful to know the density. The sandstone will be less dense (~2.6) than the basalt (~3.2). $\endgroup$ – Gordon Stanger Oct 29 '16 at 21:34
  • 2
    $\begingroup$ Hello, and welcome to Stack Exchange. My complements on the question: you've obviously done your homework. $\endgroup$ – Daniel Griscom Oct 30 '16 at 2:07
  • 2
    $\begingroup$ Thank you for asking a "what's the rock" question that actually has all the info that's usually requested. I hope you get an answer! $\endgroup$ – Semidiurnal Simon Nov 3 '16 at 21:34
  • $\begingroup$ Mafic rock, either metamorphic or igneous. Basalt, gabbro, or their derived metamorphic equivalents (either through a sedimentary intermediate or directly). Those vein looking things suggest it to be metamorphic rather than igneous, but who knows. A geologist will have to look at a thin section to know for sure. $\endgroup$ – Gimelist Nov 3 '16 at 23:30
  • $\begingroup$ density would be helpful, I have even seen beach tossed asphalt that looks like that although it would be much softer. Even a mass and something for scale in the picture would help. use a coin or pencil $\endgroup$ – John Jan 3 '17 at 22:41
1
$\begingroup$

It is always hard to identify rocks from a distance so take this with a grain of salt.

There appear to be bronze-coloured pyrite crystals in the rock that aren't usually seen in basalt which is a black igneous/volcanic rock. It must be quite well indurated - i.e. compressed to a hard, solid rock, because it held together as the waves smoothed it into the nice rounded cobble. That indicates it isn't a sedimentary mudstone or siltstone, which would likely be too soft. Those sediments can be metamorphosed by heat and pressure that make the rock harder. This looks like the metamorphic rock, slate, except that slate has a very strong cleavage so it breaks apart into sheets and can even be used to make roofing tiles.

So it is a bit of a puzzle, on one hand it looks like igneous basalt or diabase but with disseminated pyrite, but on the other like slate but without the cleavage. My best guess would be metamorphosed sediment that for some reason didn't form under the correct conditions to develop the cleavage. I think a geologist would have to look at a thin section under a microscope to be sure unless they were familiar with the area.

$\endgroup$
1
$\begingroup$

To state the obvious, a heavily rounded "tough" rock had to have been transported under some serious erosive power from elsewhere. According to glacial transport maps for the Lake Ontario area, surficial rocks of this region have their origins somewhere to the northeast, in Quebec's precambrian bedrock of the Canadian Shield and are likely to be gneiss.

$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.