Ooids
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In geology, ooids are a type of sand grains that usually form on the sea floor. After being buried under additional sediment, these grains can be fused into a solid mass of sedimentary rock, called an oolite. Oolites usually consist of calcium carbonate meaning they belong to the limestone rock family.
Similar to a snowball which accumlates more snow as it rolls down a hill, ooids form by growing larger and accreting material as they move around. (In addition to the physical accreation, ooids may gain mass from chemical precipitation.) Regardless, microbes play an important role in the growth.
Kidney stones are a type of ooid, as they are rich in calcium and possess similar growth structures.
External link
- Sculpture Conservation: Ooids and Ooites (http://www.sculptureconservation.com/oolites.html)