September 11, 2011

Geology Word of the Week: O is for Ooid

Posted by Evelyn Mervine

Ooid sand from the shores of the Great Salt Lake, Utah. Photo courtesy of Matt Kuchta.

def. Ooid:
A small (generally less than 2 mm), spherical or ellipsoidal concretion of calcium carbonate (CaCO3) that has generally formed around a “nucleus” such as a shell fragment or a quartz grain. The word ooid is derived from an ancient Greek word meaning egg-shaped. According to the Oxford English dictionary, the name came about because ooids resemble roe (fish eggs).

Some geology words I love just because they’re just so much fun. “Ooid” is one of those words. “Ooid” is a great word because it looks and sounds like the geological entity it represents. The word is oval (O) and round (o) just like ooids themselves. The word also rolls off the tongue in a squishy way that makes me think of marine ooze, which might be found in the vicinity of ooids.

Here are a few bonus, related words:
def. Oolite:
A sedimentary rock composed of lithified (“made into rock”) ooids.

def. Oolith:
A synonym for ooid, often used to refer to a single grain.

A synonym for oolite is roestone— literally, fish egg stone!

Other related oo- words are ooidal,  oolithic, oolitic, and oolitiferous. That last word sounds somewhat fake, but I found it in the trusty Oxford English Dictionary!

For more scientific information about ooids, here is a good article titled “Ooid Formation” (on a wonderfully-named website called Geology Rocks) that describes ooids far better than I could.

Here are oodles of ooid and oolite pictures:

Modern ooids from the Bahamas. Photo courtesy of Callan Bentley. Note scale in top left corner.

More modern ooids from the Bahamas. Photo courtesy of Callan Bentley. Note scale in top left corner.

Oolitic limestone from the Rierdon Formation, a Jurassic unit from Montana. Photo courtesy of Callan Bentley. Note scale in top left corner.

A jar of ooids! Photo courtesy of Paul Glasser.

Ooooo so many ooids! Photo courtesy of Paul Glasser.

Ooo000ooids! Photo courtesy of Paul Glasser.

Ooids in a petri dish. Photo courtesy of Paul Glasser.

Oolitic limestone deskcrop. Photo courtesy of Ron Schott.

Oolitic Portland Limestone, commonly used as a building stone. Photo courtesy of Ian Stimpson.



“ooid, n.” The Oxford English Dictionary. 2nd ed. 1989. OED Online. Oxford University Press. 11 September 2011.

Other words such as roestone, oolith, etc. were also looked up in the OED.


***Thanks to Christie Wilcox for suggesting this week’s word. Thanks to Matt Kuchta, Callan Bentley, Paul Glasser, and Ian Stimpson for providing pictures. Thanks especially to Paul Glasser for naming his picture folder “oodles of ooids,” a delightful phrase which I promptly stole for this blog post.***