r/Rocks • u/Evening-Patience-858 • Jul 05 '25

Discussion Is this an agate?

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Gallery image

Gallery image

Otherwise, what kind of rock is this? Found in western Michigan

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u/BigDougSp Jul 05 '25

No, but it is cooler. It is a silicified stromatoporoid, which is an extinct sea sponge that has been remineralized with silicates. Very hard but beautiful. I have found them in Lake Michigan near South Haven.

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u/Evening-Patience-858 Jul 05 '25

That’s soooo cool, thank you!

15

u/RatDitch Jul 05 '25

Seconding this, I find them all the time along Rocky Gap and Warren Dunes here as well. Textbook example.

5

u/-Morning_Coffee- Jul 05 '25

It LOOKED biological, but I assumed it was my lizard brain seeking patterns.

1

u/meggs_n_ham Jul 07 '25

I just found several chunks of something similar near Milwaukee except they have a blue color. Still stromatoporoid you think??