they sell adhesive 'stick' things at home depot, specifically designed to adhere objects to a surface for an earthquake. you could put a few of them on the bottom to hold it in place.

another idea, make a little 'guardrail' for it, paint the same color as wall, and adhere in front of it.

You could try a thin wire loop (or loops) around it and screw the wire(s)to a stud as well as the sticky putty.

Kenj, this product was suggested to me by a Houzzer here for i have place big bottles on a very high shelf in my kitchen and had the same concerns

That is one beautiful geode!

For something that heavy, I would use Velcro. (hook and loop tape)

That is what is recommended for very heavy objects.

It might take off the paint when you try to remove it, but so what, it is better than having your amethyst geode crack and break, one that size is very costly.

You might also think about insuring it if you live in earthquake territory.

The other thing is, you really do want to stick it down well with the Velcro because if it falls on the head of a guest you certainly wouldn't want to be considered negligent.

Joe’s Sticky Stuff. Super sticky and leaves no residue. Museum Putty leaves greasy residue. Gorgeous geode. To give you idea of how this works, I put a mirror onto another mirror and it worked like a charm. Mirror was around 15-20 Lbs! It is amazing stuff and leaves no spot when removed. I get mine on Amazon.