QUESTION: My grandkids have an interest in geology and the question came up what is the rock used in our flower beds. I believe it was labeled "River Rock" and walnut was mentioned. There are parts that are very smooth so I was thinking it was either siltstone or chert, however when fractured, the fracture area under a lens looks like fine grain particles and not shiny like I would expect with chert. It is not obvious whether or not the particles themselves are fractured or remain intact like a true sandstone siltstone. The fracture as you see is more angular and not conchoidal. I have another rock that is similar and it has quarts veins in it. Does the presence of quartz veind indicate it is more likley a hornfels? Thanks for any help you can give.
ANSWER: PL. scratch it with (i) a piece of glass and by the edge of a small knife blade - is it scratch able
(ii) if not, it could be chert. If yes, it could be a carbonate concretion? - to check this pl. add a few drops of dilute hydrochloric acid on clean surface of the sample, if it gives brisk effervescence, it could be carbonate.
remote identification could be misleading; besides pics, some physical attributes are required to identify rocks and minerals.
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QUESTION: It took me a while to get Hydrochloric Acid. The rock scratches glass and is not scratched by a knife. It also does not fizz with HCl. when it breaks it is not smooth but the entire outside is very smooth.
ANSWER: Thanks for the follow up
from your description, I feel, two possibilities :
(1) Man made material for laying in flower beds.
(2) (you say it appears to be very fine grained under lens)- I feel it may be very fine grained quartzite pebbles, I have some reservations in calling it to be a variety of chert/opal/chalcedony etc. because of your additional info about the grain size and nature of fracture (not Conchoidal)under lens. Would it be possible for you to find out its sp. Gravity- Quartz varieties inclusive of fine grained quartzite should be around 2.65
the Second similar sample - presence of quartz vein does not a necessary indicator for a rock to be called a hornfels.
A hornfels is basically a contact metamorphic rock - essentially fine grained but with presence of some metamorphic minerals ( relatively coarser than the grains of the rock) randomly oriented.
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QUESTION: The rocks density is 2.60, so it is entirely quartz based? Does that help? Is it a microcrystalline siltstone or chert? I have included another example of the rock type to show the fracture. There are also vugs in it.
At a place where the sample is chipped, the rock appears to be like chalcedony and the ocherous colour seems to be surficial coating.
(1) Kindly do one more experiment, keep the sample immersed in concentrated hydrochloric or sulphuric acid for about 20-30 minutes and then drain the acid and wash the sample in running water - pl. check if the colour (stain) disappears (pl. take adequate precautions while handling acids, it should be done preferably in a geology or chemistry lab. under supervision of some teacher.
(2) If you can afford to break the sample by a hammer you might see the inside colour of fresh rock.