User blog:Hugues de Payens/Heimdall's Gem Theory

I think Heimdall's gem is meant to represent Asgard. In Thor, Thor drew the Nine Realms. The Nine Realms were in the form of Yggdrasil, with Asgard at the top. In Thor: The Dark World, Heimdall's armor featured Yggdrasil, and the gem is in the location of where Asgard was shown on the map of the Nine Realms.

So, I think the gem represents Asgard and it gives his armor a patriotic meaning.