@Enlong, regarding your post a few pages back: Bear in mind that the crystaline aspect doesn't seem to be 'unique' to the Crystal ponies, since the rest of the cast had it at the end as well. In fact, it seems to be an aspect of the heart - it's possible that their connection to the heart was simply made so strong by generations of being near it that they developed an inborn line to the heart. So while the girls had to be exposed to 'heart' radiation, the crystal ponies are born naturally irradiated.
I can't say for sure that the heart is tied to the specific tribe. After all, the magic seems to work for all of them. To be honest though, Considering Fluttershy's basically got EP magic going on and seems to put animals at ease, and Pinkie's skill at making people... happy... not to mention AJ's motherliness. It wouldn't surprise me if an element of the Earth Ponies 'connection to the earth' was also a connection to the things
of the earth - ponies included. In other words, 'mood magic'.
What I'm getting at is, I think the EP's may be the cause and not the effect. I think it's possible that their presence, the concentrated presence of nothing but Earth Ponies, forged the heart itself. A concentrated gem of mood magic.
Or, in reverse, the huge amount of EP magic coming from the stone could have altered the ponies around it, eventually mutating all of them into EP's over several generations. Which is probably more likely, since it explains their connection to the stone in a way that also explaions why then Pinkie and AJ and fluttershy didn't get connected to it instantly as well.
In short? I don't think Sombra has anything to do with it. Whether it's the cause orthe effect, the Heart Stone
is connected to the citizens on a very intrinsic level. It's magic is a part of them, and considering how deeply a pony's magic seems to be tied to their species(Barring a few oddities), I don't think it's an accident that they're all of the same species.