Oolites: they're made from ooids!

Posted by Maranda Powers on

Researching this blog post led me down a big ol’ oolitic rabbit hole, and introduced me to some very fun words!*  

First cool word - ooid. An ooid is a spherical grain made up of concentric layers of minerals. They are usually smaller than 2mm, if they grow larger than that they become giant ooids - very scientific!

To make an ooid you’ll need a shallow sea with lots of dissolved minerals hanging around. It’s not necessary, but often a little bit of organic material like a shell fragment starts the process, and minerals begin to deposit around it, building up evenly, layer by layer (picture a jawbreaker). The key to getting these nice and round is water movement. Tides, currents, or consistently stormy or wavy conditions will all work - what’s important is that the water is moving, continually rolling our little ooids around to get even deposition of minerals all around. In short, the water is moving, the ooids are rolling and the minerals are depositing!

Left: modern day ooids from the Bahamas, about .5 mm. Right: Chinese oolite carved into a decorative sphere.

All good things must come to an end however, you can’t just roll around in the water getting nice and plump forever! Eventually conditions change, the water stills, and sediments begin to cover our lovely little ooids. The sediments build, the pressure increases, time passes, minerals solidify and our ooids have become...oolites!

So ooids are the individual layered spheroids and oolites are a sedimentary rock consisting of many ooids or ooids in matrix. 

A magnified image of oolitic grainstone showing the layers of deposition. (source)

Agatized or jasperized oolites are used in jewelry making because they contain enough microscopic quartz crystals to make them hard enough to take a nice polish and often have appealing colors, but most oolites don’t fall into this category. For example, oolitic (cool word #2) limestone is a more common occurrence. It is found in several parts of the world and forms when the sediment that covers the ooids is mainly calcium carbonate. Now here is the fun part...SO MANY buildings are made from oolitic limestone! Perhaps you’ve heard of the Empire State Building or the Pentagon?? Talk about a fun fact! The building material for both of those and lots more buildings came from a little town called Oolitic, Indiana (Do you live here? Send me a t-shirt ASAP!). This was the main rabbit hole I fell down, and it was worth it. Plus it led me to an Atlas Obscura article and that is always a fun place to be! 

Empire Quarry, by Atlas Obscura user Cvllrll

The aptly named Empire Quarry in Oolitic, Indiana supplied 18,630 tons of stone for the Empire State Building. Now it is no longer an active site and has filled with water - it looks so beautiful and so terrifying and I’m so bummed that it is private property. Judging by how many pics there are on the internet, it seems that folks do visit, but I am a rule-following chicken. Although it’s only a four hour drive from Chicago…..but nope, too scary! If you’ve been - please tell me all about it!

When choosing stones to work with, I’m generally drawn to the ones that you can “see” the geological processes that made it, for instance I am much more drawn to malachite and it’s striations showing the layers of mineral deposition than I am to stones like jade or aventurine that are solid colors. Don’t get me wrong, I love all rocks and they all have a story to tell - some just spark my curiosity more than others. Oolites definitely speak to me in this sense - they are both visually interesting and geologically interesting. I see many more oolitic research rabbit holes and oolite collections in my future!

You can click here to see the collection that inspired this post, or feel free to reachout and see if I have any more oolitic lovelies in the works.

As always, thanks for reading!


* I looked it up, oolite, oolith, and oolitic can be used in Scrabble, but not ooid. 

P.S. The oolites I used in this collection came from China, but I didn’t have any info about a specific locality so I set off on a little internet journey to see if I could figure it out.

Spoiler: I did not.

But I did end up reading a few papers about giant oolites formed in China during the Induan age (251.2 - 251.9 million years ago), right after the Permian mass extinction. And *maybe* understanding some of them?? Here’s what I can figure: the lack of plant and animal life in the sea, geologic factors from the breaking up of Pangea and extra stormy weather from intense climate events seem to have all converged to make an exceptional ooid forming environment, which was then covered and preserved and now millions of years later scientists can study it and describe it and use it to piece together the amazing history of this planet! Science is amazing and rocks are the coolest!! 

This is one of the papers I read, if you are a very smart person and want to read it and tell me I’m wrong, please do! Nicely…. :) 


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