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A while back (pre halloween of this year), I predicted that Gems would rise to 10 gold by Christmas. Well, it's starting to look like I was wrong, as they could easily surpass that number! Even though gems have obnoxious taxes compared to all other transactions, they make you a lot of gold, provided you bought in back when I told you to on the secret forums (link goes straight to the topic).
People have pointed at different events and specific items for sale on the gem store as reasons for this increase. But they're all wrong, and I'll prove I'm right by summer time.
What causes gems to rise in value? The high school players. They get more and more time off at this point in the winter as well as during the summer.
From October to November of last year, gem prices steadily rose, before reverting to a much slower pace from January to June. July to October saw a little bit quicker rate of growth, but then things once again exploded during October.
ArenaNet has statistics on when the most people will play their game, and they act on that information by providing deals on the gem store and releasing new content.
Gems will always continue to grow in value as long as they update this game, so you're best bet is to buy them either right now or in January, then sell a year later during the Christmas holiday.