Have a group of Dwarves surrounging it. Put it free-standing on logs like they are moving an monolitic piece, like how the ancients moved the Stonehenge rocks or the pyrimid rocks. That way the stone is untouched and can be removed from the diorama.
I like this idea. Makes it very easy to look at the sample more closely.
Riffing off earlier ideas, I think it would be awesome to see the rock standing vertically, with dwarves on one side in their meticulous planning stages with their precision tools, maybe a camp table set up with some plans spread across it, some scaffolding in process, etc...and on the other side of the vein, a group of goblins with their dynamite and torches, maybe one guy on a rock trying to pull it out with his bare hands (or teeth). I don't know if your sister would appreciate this concept, but it's a fun little dio in my brain.