Hi!
When you first come across those gems you should have seen a brief cut scene that showed one turning into a fire elemental. I'm not sure what the trick is but as I wandered around with them in my inventory they eventually do just that, turn into fire elementals...perhaps when I walked close to the flames from the ground this triggers the change? Anyway, that accounts for them 'disappering' from your inventory.
Tal'Missra
