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it can be both
A grain can be a tiny piece of rock, yes. A grain can also be a grain of salt, meaning one 'pellet' of salt.
An allochem is a grain of rock in a carbonate rock.
Grain refers to particle or crystal size in rock.
Marble is has no grain so it is classified as a glassy rock.
the pattern in a rock
Gneiss has a small to medium grain size.
It depends on if it is intrusive or extrusive. If it is an intrusive rock, it is formed when magma hardens under the earth and has a larger grain size. If it is an extrusive rock, it is formed when lava hardens on the earth's surface and has a smaller grain size.
The cooling rate of the rock.
A porphyrytic igneous rock.