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From Wikipedia:

Rock or stone is a natural substance, a solid aggregate of one or more minerals or mineraloids. For example, granite, a common rock, is a combination of the minerals quartz, feldspar, and biotite. The Earth's outer solid layer, the lithosphere, is made of rock.

From Wikipedia page about gallstones:

A gallstone is a stone formed within the gallbladder out of bile components

So if rocks are stones, what type of rock are gallstones?

  • Igneous rock is formed through the cooling and solidification of magma or lava.

  • Sedimentary rocks are types of rock that are formed by the deposition and subsequent cementation of that material at the Earth's surface and within bodies of water.

  • Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism

Gallstones don't fit into any of these categories so what type of rock is it?

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    $\begingroup$ Could also reasonably call it an English question or an example of using a less sharp definition. Simple Wiki says a tree is a tall plant with a trunk and branches made of wood. Doesn't mean a family tree is such a plant :-). Indeed, sources that aren't true full dictionaries will pass on non pertinent definitions. Doesn't make the word valid. If you wanted to ask on the English SE, they might well be able to explain where the connection first came about $\endgroup$
    – JeopardyTempest
    Commented May 26, 2017 at 12:38
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    $\begingroup$ @JeopardyTempest: For instance, it's common (at least in gardening circles) to talk of "stone fruit" like peaches & plums, which have a hard stone or pit in the center, even though that "stone" is vegetable rather than mineral. $\endgroup$
    – jamesqf
    Commented May 27, 2017 at 5:47

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The nature of this question is more about biological chemistry than it is about earth science. This is because gall stones have nothing to do with geology, but chemical processes in the body.

Consequently, it would be inappropriate to apply geological classifications, such as igneous, sedimentary or metamorphic, to gall stones.

Due to the way gall stones are formed - layers of bile coating a nucleus - they could be describes as being concretions.

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  • $\begingroup$ I suppose a body's stone really isn't ALL that different from sedimentary silting. It's definitely not a result of temperature and pressure! $\endgroup$
    – JeopardyTempest
    Commented May 26, 2017 at 12:39
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In the geologic sense of the word, rocks or stones are made up predominantly of inorganic minerals. There certainly are rocks like some limestones that contain a large proportion of biogeochemically produced minerals like shells or later alteration of the original biological material. Some rocks may contain some organic matter but it often doesn't survive the process of compacting and lithifying (turning into rock).

However, from the Wiki page, it sounds like gall stones are dominantly organic matter like cholesterol that may contain some inorganic material like calcium phosphate or calcium carbonate. So I wouldn't really consider them stone in the sense used in earth science but maybe there could be some gall stones that get trapped in sediments after a person dies and then turn into a tiny proportion of a sedimentary rock.

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"Metamorphic rocks arise from the transformation of existing rock types" "a natural substance, a solid aggregate of one or more minerals or mineraloids."

Looks like Metamorphic wins.

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