You can obtain all skill gems from drops. Some with quality some without, but you would be really lucky to get all of them, maybe in a few years
. The best way to get most of the gems is by leveling all exiles to 60 with all the quests done. I never really had any problems getting gem's because I trade with friends. Magic find will increase the chances of your drops, but chances are you will just get rares and above. Also new gems are added every patch, so will need a big stash if you want to have them all.