Hot answers tagged

8

This is the key: used to be part It is important to look at this from a historical point of view. Up until the 1950s and early 1960s, there was no agreement to how granites form. This became the granite controversy. On the one hand, experimental work showed that granites crystallise from magma, but this magma had to be derived from basalt-like magmas. ...


8

That's amethyst, a violet version of quartz See here


8

Your rock is most likely colonial horn coral fossil. It is common fossil in middle Devonian Period in Kentucky. I won't venture a guess on species from just an image. Kentucky Geological Survey has a nice introductory guide to fossil corals.


7

Even rocks that have lasted for billions of years decompose when exposed to Earth's weather. What you have is indeed granite, but a somewhat decomposed granite, which is treated by geologists differently from the intact, solid stuff. The "crumbly bits" have their own name: grus. Granite is made of crystals of different minerals (mostly feldspar and ...


6

Yes, this is a granite. It's hard to know exactly which one. A geological map of the area shows the bedrock to be sedimentary rocks, so this piece of granite was transported in the creek from far away (but not too far, otherwise it will crumble completely). If we go further east, for example with this geological map, we find that: To the east, the ...


6

You live in an very good state for interesting geology, rock collecting, etc. In regards to looking for things the kids can collect, view and otherwise analyse the PDF, "The Collectors Guide to Rocks, Minerals, and Fossil Localities of Utah" would be a good start to your research. The publication is from the Utah government and covers a large number of ...


6

That’s labradorite, it’s a silicate and a type of plagioclase feldspar named after Labrador, Canada. The iridescent colours that are seen is called the “schiller” effect. It is caused by light being scattered by thin layers of feldspar within the rock that formed as it cooled.


6

This is man made glass. The fracture pattern is characteristic of glass, and no natural glasses that I know of have this colour. The area where you found it is well known for its glass making history. From the Wikipedia page: Streator is a city in LaSalle and Livingston counties in the U.S. state of Illinois. The city is situated on the Vermilion River ...


5

The mottled blue appearance of the host rock looks very much like a rodingite or serpentinite. These rocks appear in ophiolites, and Macedonia has many ophiolites. Discard this rock. It may contain asbestos.


5

Your rock is a classic example of chert, one of the most common rocks on the surface of our planet.


4

Your rock is almost certainly a form of quartz - either quartzite, which is metamorphosed sandstone or vein quartz, which is deposited by hot solutions underground. I think probably quartzite. Notice the grainy fracture - some minerals will break along flat surfaces but quartz does not (you may see flat surfaces from the crystal growth, but not from cleavage)...


4

This is a so called Moqui Marble. A spherical concretion formed by sand and iron based minerals like limonite. Further information can be found here


4

Main difference between a subvolcanic verses a plutonic is depth at which the rock solidified at from its molten state. Plutonic implies a depth greater than subvolcanic by definition but I am sure there is some overlap between shallow end of plutonic and subvolcanic. Depth of emplacement for plutonic would mostly be much greater than 2 km. Plutonic is ...


4

Limestones are usually categorized following the Dunham & Folk classification. Only very rare or special limestones were given a name and these are often named after the location where they can be found e.g. Travertine. The images you provided are not very explicit which makes it literally impossible to clearly identify the limestone. Please consider ...


4

They are not synonymous. Cleavage means breaking along planes defined by crystallographic directions. For example, cubic crystals like halite, NaCl, often cleave along directions that follow the cubic form. However, with a different bond strength, like fluorite $\ce{CaF2}$, the crystals cleave most easily along octahedral directions, similar to two ...


4

Chlorine is a very reactive element. Contrary to your statement, chlorine occurs in many rocks and minerals. The mineral apatite, with the composition: Ca5(PO4)3(F,Cl,OH), is a natural source of chlorine. Virtually all rocks contain chlorine. Chlorine even occurs occurs in alkaline magmas. One of the most common minerals containing chlorine is common ...


4

Looks like Quartz (massive transparent/translucent crystal or rock without cleavages and hardness of 6) with iron oxyhydroxide coating, hence the red color. Could also be feldspar there, and then it would be an alkali granite. Most likely product of a late magmatism, as a vein or something similar. (if my identification is correct)


4

It looks to me like a type of flint. Flint nodules are found in chalk and some types of limestone. By banging them with a hammer, you should be able to strike off curved chips with sharp edges. Fractures of flint typically show a conchoid (shell-like) curve. The interior of a flint nodule is brownish, with a glassy texture, but the exterior is often white ...


3

Given the lighter colored fine grained matrix material with the dark crystals I would suggest that this rock is extrusive with phenocrysts of hornblende (a dark version of an Amphibole). Without addition information about the matrix material or the phenocrysts I would class this rock as a Andesitic hornblende porphyry. See the following site for more ...


3

Your rock is composed mostly of quartz based texture of the rock's surface and color. It is not jadeite nor nephrite. Mutton fat jade is typically more pale yellowish in color.


3

We have a guide on how to ask a good "Identify this rock" question (sorry it isn't easier to find. It gives you the kind of information that helps us provide better answers and helps teach you the sort of things that will help if you want to learn how to answer your own questions. From your picture, your rock appears to be made up of the mineral quartz (...


3

This one is a simple concretion. The interior is mudstone, it is rich in iron, lending to it's orange-brown color. They are dense, very common and often made from a small organic material (a piece of a plant, shell or animal) rolling around in sediment and accreting material in a concentric pattern. Hence, when it hardens into rock and thus breaks, it breaks ...


3

The iridescence and colours make me think it’s chalcopyrite peacock ore, a copper iron sulfide. Chalcopyrite is yellow but the blues and purples are caused by iron and copper within the chalcopyrite becoming oxidised.


3

I belive what you have found is reflective road paint,The color is similar to road markings and it do contain glass beads. It does not look like any natural rock i know of but it do look like this type of material https://en.wikipedia.org/wiki/Thermoplastic_road_marking_paint I am not saying this is what it is but it is one possibility so please forgive ...


3

You have a metamorphic rock that looks primarily to be a mica schist with possibly a band of gneiss.


3

Your rock appears to quartz stalactite with druzy quartz crystals coating a banded chalcedony. A nice little find but is probably not really worth much. Label it and put in a perky box, the specimen might fetch $1-2 US dollars. Quartz stalactite images.


3

Your rock appears to be quartz var. chalcedony (jasper) with the interior being quartz pseudomorph after another mineral probably calcite or aragonite. I've seen a number of quartz pseudomorph after calcite from Indiana over the years but I do not have a good reference for Indiana pseudomorphs.


2

I own this stone which was given to me as dinosaur egg (I'm vertebrate paleontologist). It weighs 2.9 kilograms and shows something shell-like. It's no dinosaur egg or something like this, it's a simple iron rich concretion which shows some shell-like erosion structures.


2

This is most likely a piece of greywacke with some pieces of quartz. It's been rounded by wave action (or also river action, since it may have been carried to the sea from where it's exposed in the mountains). Greywacke is New Zealand's most common basement rock ; it was deposited in a giant fan off a continental shelf and then scraped up onto the Australian ...


2

Quite an interesting stone. I cannot identify all of it but given its location, and the patterns it is primarily Agate, a type of crypto-crystalline quartz. The fourth picture down shows a layer of lace agate across the top of the stone at that location, it is underlain with a more massive translucent agate. In the Oregon area there are many areas with ...


Only top voted, non community-wiki answers of a minimum length are eligible