I am assuming that you have that button set to Always Show and when an Always Show button has no items, it shows a default icon. For Healthstones, that default icon is a Healthstone. That's why it'd dimmed and has no tooltip. So that's 1 part of the mystery.
The other part is why it thinks you have no relevant items, and I'm a bit puzzled by that.
Can you create a macro and past this text into it:
/dump LibStub("LibPeriodicTable-3.1"):ItemSearch(({GameTooltip:GetItem()})[2])
Then drag that macro on to your normal action bar. Open your bag and hover over the healthstone and hit the hotkey for the macro. It should print out some stuff that might be helpful.