Maybe a venomous biting vermin with [VERMIN_MICRO] and flier so you get swarms of them (like flies) and can't kill them. They could be called killer particles or something like that to make the illusion that they're toxic dust or something.
Edit:
Um. You can just use 'k' to see what the wall is made of after getting the announcement.
Wouldn't it just say it was a rough death wall for all three types?
If the killer death stone is hidden in safe death stone. I think he meant he hid teal death in microcline and yellow death in orthoclase type of thing. Can't really be having usable death stones. This is a death block wall.
"Your miners discovered yellow death price the miners!" then player put k and cautiosly looks one by one wich one is the death stone 
I mean that if the stone was hidden in marble, I would go and change the ingame name to marble, and also change the color and symbols to match those of marble. Then, when you strike it in game, you would be mining in marble and just get a message, you have struck marble. And if you 'k' over to it, all of the walls will appear the same.