I found a beautiful chunk of rock on a hike recently that an app identified as granite – I thought it may be marble due to the marbling of the colors within. But when I try and look up the same question as above, I’m just getting results about how to tell if a (countertop) is granite or marble. Nothing on simple outdoors rocks themselves.
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Ok, so the first thing to establish is that the names for rock types as used by a geologist and rock types as used by people making countertops, while they (sometimes) use the same words, often have only a vague relation to each other. This is kind of covered a bit in posts like this one from a company selling countertop materials (perhaps in response to angry geologists sending them messages telling them that nearly none of the things they call "granite" are actually granite in a geologic sense). In general, sometimes a "granite" countertop is actually a granite and sometimes a "marble" countertop is actually a marble, but very often, building materials described as a "granite" or "marble" would not be called as such by a geologist.
I honestly have no idea how exactly different types of commercial stone is classified, but I assume it’s largely based on some mixture of appearance (i.e., what color is it, etc.) and physical characteristics (i.e., how hard is it, how porous is it, etc.), but I’ll leave that to someone else with more expertise in that realm to explain. In terms of how we classify rock types in geology, things get complicated, fast (which is probably why people making countertops lump things into broad categories based on either aesthetic or engineering properties). At the simplest sense, rock names typically reflect some mixture of composition (broadly defined), texture, and how the rock was formed (i.e., is it igneous, metamorphic, or sedimentary).
What this means is that you can have rocks that have the same effective composition (and I’ll get to the different things that might be implied by "composition" in a bit), but that have different textures and/or were formed through different means that will all have different names. For example, a granite, a rhyolite, an orthogneiss, an orthoschist, and an arkosic sandstone might all have effectively the same composition, but they are named differently to reflect both formation mechanism and aspects of texture. I.e., a granite is a course grained igneous rock, a rhyolite is a fine grained igneous rock (i.e., a volcanic rock), an orthogneiss is a coarse grained and banded metamorphic rock where the ortho- prefix indicates it was formed by metamorphism from an igneous rock, an orthoschist is a more fine grained and platy metamorphic rock, and an arkosic sandstone is a sedimentary rock composed of sand-sized grains with a mixture of quartz and feldspars (and yes, for any igneous and/or metamorphic petrologists out there, I know that the usage of the "ortho" prefixes is a bit outdated, but it remains a useful way to generally indicate that a given metamorphic rock has an igneous protolith).
The composition part of rock type can mean a couple of different things, but in general we try to define rock types such that regardless of how you measure composition, the same name is applied (considering the textural and formation mechanism bits first). For example, for a coarse grained igneous rock that you’d call a granite, you might determine it’s a granite from either field identification (i.e., estimating its "modal mineralogy" from a rough estimate of percentages of types of minerals you can observe in handsample or an outcrop) or petrographically (i.e., you cut a very thin slice of rock, mount it on a slide, and look at it under a microscope to identify individual minerals and estimate its modal mineralogy) where you classify it as a granite based on the relative percentages of potassium feldspar, plagioclase feldspar, and quartz using a diagram like this and where to be complete, you’d probably want to further clarify whether it was a potassium feldspar granite, a syenogranite, or a monzogranite. You also might classify a granite geochemically by taking a piece, homogenizing it, and analyzing the elemental composition and plotting it on diagrams like these, where the upper left is a common way to classify different coarse grained igneous rocks based on their weight percent of silicon, potassium, and sodium oxides, but where there are many other ways to geochemically classify (and sub-classify) granites and other types of rocks. In theory, a particular rock classified in hand sample vs petrographically vs geochemically should yield the same rock type, but in practice, the definitions don’t perfectly align for a variety of reasons.
Getting back to the original question, from a geologic perspective, both granites and marbles are very much real rock types that 100% exist in nature and both of have somewhat strict definitions and in fact are quite different in nearly every way we classify rocks. I.e., a granite is a coarse grained igneous rock crystallized from a melt that was rich in silica whereas a marble is a metamorphic rock, usually relatively fine grained, formed from metamorphism of a carbonate rich sedimentary rock like a limestone (and for any carbonate petrologists out there, I’m not going into any of the ways we classify limestones because it gives me PTSD from the carbonate petrology course I took in grad school). Without a picture or location information, neither I or anyone else is going to be able to help you identify what the actual rock type of the rock you found might be. The description of it maybe being granitic based on color (which I’m guessing is maybe what the app might be using, but you’ll notice that color was not one of the things we used to classify rocks, but for an experienced geologist, we often use color to help us narrow things down a bit), i.e., granites are often on the gray side with individual minerals that tend to be gray, white, pinkish, and black, but having a "marbling" (where that is just a term that is used to describe a kind of mottled or swirly texture irrespective of it having anything to do with "marble" as a rock type, e.g., describing the "marbling" of a piece of meat does not imply said meat is a metamorphosed chunk of carbonate), would lead me to guess it may have actually been a gneiss, but again, without a picture or location information (or ideally both), that’s just a wild ass guess.
EDIT: It’s probably way more information that you (or most people reading this) want, but if you’re interested in trying to learn how to "name" rocks in the field (and doing so how geologists would, not people making countertops), guides like these are aimed at how you would go about naming rocks based on characteristics you can observe in hand sample and that don’t really require any special equipment (beyond maybe a rock hammer and a hand lens). Even with these, it’s often hard to learn how to do this without basically seeing a bunch of examples, which is a great excuse to take an introductory geology course (ideally with a lab, where the lab is where we usually really focus on rock and mineral identification).
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Marble is a metamorphic rock composed of calcium carbonate (limestone) that has then been buried and squeezed by geologic processes. It is a real rock, and usually what’s sold as marble is real marble. For example, a coral reef could later be buried, then squeezed by plate tectonics and you get marble.
Granite and especially “quartzite” are real rocks but mischaracterized in the countertop industry. Granite is an igneous rock formed in the magma chamber consisting of silicate minerals. Quartzite is a metamorphic rock formed from burial of quartz, usually compressed and mildly heated. Neither granite or quartzite have “banding” or foliation, meaning they appear granular. They form very differently, but both can be metamorphosed (heated and squeezed) via plate tectonics further changing the rock. They can form different meta rocks like gneiss or schist.
If it’s actual granite, they will call it granite. But they will also call gneiss and schist granite. There is also a thing called pegmatite which is like granite but forms larger gem quality crystals. I scored getting a beautiful pegmatite with gem quality quartz and garnet they had priced as the same as all the other “granite.”
I know you didn’t ask about quartzite but they will sell quartzite and high quartz content gneiss as quartzite.