Capcom has updated their Street Fighter X Tekken website（http://www.capcom.co.jp/sfxtk_en/system.html） with a lot of information about the gem system. They talk about the different kind of gems and provide images showing them in game, there is also a list of all of the gems currently.
Assist Gems primarily support the player's ability.
An example of this are "Easy / Super Easy Inputs". Using these Gems, anyone can easily unleash special moves! Look at how Ryu's basic special move commands change.
