GX000 posted...thank Celestia that the elements of harmony don't need their owners around to activate them...
you dont need them to activate the stones, but you DO need the characters to complete the story, apparently.
so far the best way of obtaining gems I have come across is the 'get free gems' option that occasionally shows itself on the gem purchasing page. I've only seen it once, though - I got 10 gems for watching 10 advertisements, and then they ran out of ads. I havn't seen the option since.
the second best way is getting them as bonuses for levelling up, completing quests and logging in every day - though of course, thanks to level cap at 60 this source of revenue is finite. At 3 gems every 5 days with the daily reward system, it would take you almost 3 years to save up for both Rainbow Dash and Rarity.
Though I haven't utilised it myself, another method is the balloon pop game that uses heart tokens. I'm still saving up my hearts for other ponies, but apparently one 30 token game can possibly get you 2-3 gems.
personally, I'm holding out for a game update that makes things a little more fair. no way I'm forking out enough money for a real console game just so I can access a couple of characters in this one.