Interesting! This at least suggests that Crazy Eights and Emerald Miner were real.

Your theory the gem is a weapon though- control of the weather is of course a force to be reckoned with. I have no doubt it could be used in an aggressive manner as a weapon. But do you really think that these ponies would create three gems with the intention of using them as weapons and give them away to other countries? Even if they were allies, why not keep one of the three to defend your own nation?

It seems strange also that leaders would trust the creators of an evil amulet to make something meant to ensure the survival of others.