If rocks are stones, what type of rock are gallstones? 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?
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.