The story gives them a chance to be natural... so... I have to go with Jeremy's post that a lab report is a good thing here.
AGTA would be the choice for 'color' in the US. Although GIA is the giant institution in the branch... because of their ever successful diamond certificates. But again, that's one unrelated debate and whichever lab says 'natural' it'll be what they say.
If they are natural, that's f-a-b-u-l-o-u-s!
And I am not saying they are... I wouldn't have guessed from the pictures without the story. No expert here.