Item is a big stone embedded near the peak of Mt Everest. It works only if the stone is touched by bare hands. If/when this happens the mountain suddenly drops/sinks by approximately 5cm in height, causing avalanches and localised destruction.
How would someone have figured out that the rock did that? If I recall correctly, you can't have bare hands up that high, it's too cold and you'll give yourself frostbite. Also you might freeze your hands to the rock, which would be a terrible thing. Also, trying to cover-up a sudden drop in the height of such a famous mountain would be a nightmare, although it's tall enough that it might go unnoticed for a bit. I'd probably go with one that's lower, and less traveled/less famous, just for ease of containment. It would be a PR nightmare trying to keep people off Mt. Everest, after all!
Parts of the rock have been removed for testing, and this effect has not yet been reproduced outside of the original environment. The pieces must be stored at a very low temperature as they are prone to melting into an unknown liquid, and quickly evaporating otherwise
The stone is a very eye-catching colour, and is composed of as yet unidentified material.
I'm not the only one who's getting very, very tired of "unknown liquid" this and "unknown material" that. Make the rock something like, say, cinnabar. Cinnabar contains mercury, which becomes liquid at room temperature. Cinnabar is a result of recent volcanic activity, which means that there's no way it should be up on top of a non-volcanic mountain. And finally, cinnabar absolutely does not cause mountains to get smaller. It changes it from "weird rock that makes mountains shrink because it's made of weird stuff" to "this should not make the mountain shrink but it does, what the heck." Also, of course, mercury toxicity is some serious stuff caused by, you guessed it, improperly handling cinnabar.