striatic is definitely right. i sell ore and gems to vendors and buy items to donate to giants, specifically crystal chambers (as striatic mentioned). mix in some shrine powder, and BOOM! instant learning.
when i'm on a serious mining tear (specially in these last few test where I don't have many bags yet) I have been selling everything but Diamonds and Luminous Moonstones to the caverns tools vendors because the inventory slots are at a premium and filling a bag with Amber and trying to go to the shrine I need is a waste of effort.
There's a lot to consider when talking about efficiency - how much ยข is spent on food/energy/drinks, do you want to get other quests done or badges completed (smelting/crushing), where is the shrine and are you going back to mining or something else -- i know I'm not doing things the MOST efficient way, but questions like those of what to do with ore I don't fret too much and handle on a case by case basis and don't worry too much about the theory.
you actually ARE doing things the most efficient way : ]
you want to sell those higher value gems as well. sell everything.
there's nothing special about gems. they're just high value.
but there are high value tools as well, which you can buy with money.
so convert to currants to save slots, then when you want to donate take your sales and buy smelters or crystamalizing chambers or whatever. you'll get the same value back.
and remember to sell gems only to the masked vendors, who pay the most.