GryphonStar5678 
"[@Background Pony #04EE":](/images/2132960#comment_8765235
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By that moment it was clear the writers were simply just trying to end the episode. Also it was never clear what that throne was made of nor that Sombra could create exact types of gems or else he could have just created his own version of the Crystal Heart as well, or ever his own elements of Harmony. heck by the logic of simply creating the right crystal Sombra could literally forge his own Tree of Harmony with ease. Such thought would be too stupidly over power so I doubt the writers ever considered such a possibility.
Bearded Dialogue Master
By that moment it was clear the writers were simply just trying to end the episode. Also it was never clear what that throne was made of nor that Sombra could create exact types of gems or else he could have just created his own version of the Crystal Heart as well, or ever his own elements of Harmony. heck by the logic of simply creating the right crystal Sombra could literally forge his own Tree of Harmony with ease. Such thought would be too stupidly over power so I doubt the writers ever considered such a possibility.