Look out for the future, because you never know what it might bring…
Question #90944 posted on 04/02/2018 4:30 p.m.

Dear 100 Hour Board,

Could kidney stones be considered a sedimentary rock?

-Oscar E. Meinzer


Dear Oscar,

When I first saw your question in the inbox my immediate response was, "Haha, gross," so congratulations on giving me something gross and interesting to research.

First of all, let's talk about what kidney stones are, and what sedimentary rock is.

Kidney stones are made up of calcium oxalate, calcium phosphate, uric acid, and struvite, with calcium oxalate being by far the most dominant component (about 70-80% of most kidney stones are made of calcium oxalate). Sedimentary rock is made up of silt, clay, sand, and gravel that have been compressed together. Alas, the fundamental components of sedimentary rocks doesn't include calcium oxalate. In fact, except for calcium oxalate, which sometimes forms as precipitation on the sides of vats used to brew beer, all the major components of kidney stones are really only found in living organisms, not rocks. Also, sedimentary rocks are made up of relatively large components, whereas kidney stones are made up of tiny crystalline minerals that aren't even visible by themselves.

Not to be dissuaded that easily, though, I decided to compare the formation processes of both kidney stones and sedimentary rocks to see if there are any similarities.

Sedimentary rocks are made when sediments build up, layering on top of each other, and then are subjected to extreme pressure, squeezing them together into a solid rock. Kidney stones are made when the crystalline elements in urine stick together, either because there's not enough fluid in the urine to dilute them, or because the urine lacks the substances to prevent them from sticking together. This is sort of similar to the formation of sedimentary rock, in that the components build up and are then bound together, but unlike in the formation of sedimentary rock, kidney stones don't require extreme pressure.

So, based on their components and their formation processes, I don't think there are enough similarities to really classify kidney stones as sedimentary rocks, sorry. 



Dear The Grouch,

Oscar E. Meinzer asked all of us
a question, which, answer we often must
but you must know this fact
we just can't be exact
for we are not nephrogeologists

 - The Earl of Limerick