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.