[lochaber]

I may be wrong on this, but I'm pretty sure most geodes are quartz or some color variant (amethyst, rose, agate, etc.), which are all perfectly inert and safe for aquarium use.

There may be some concern over what the geode formed in, if there are any carbonates on the outer shell, etc.

If you can post a pic of the non-crystal part of the geode, that might help.

If it does have some stuff on the outer part, you may be able to give it a soak in some acid for a while, and then several freshwater rinsings to get rid of it.

tl,dr: A geode will probably be fine.

 

[lochaber]

thanks for the pic.


I can't say for certain, but the inside xls look pretty quartz-like.

If you have some sort of acid, I'd try dripping a bit of it on the outside, especially the lighter colored patches.

If I remember correctly, I think most geodes form out of igneous rocks (which are generally ok), but there are always exceptions, and there are a lot of cases where carbonates (or other somewhate soluble minerals) can get deposited amongst otherwise inert rocks.

I'd like to say it's excessive worry, but I'd still want to acid-test the outside of that geode. I'm pretty sure the inside is fine.

If you can't be bothered to test (or if it seems bad and/or ambiguous...) you could always coat the outside with resin (and, considering what HighDesert said, the cut/polished face as well), which should render it aquarium safe after it's cured...