Altho' Ms Markle is descended from Africans [as ultimately we ALL are], that descent is in the remote past.
It would be like giving Lady Gabriella Windsor Siberian Emeralds as a Wedding present, because she has distant Russian Ancestry...
American Gems would be rather more pertinent to her as an individual, surely ?
I think the suggestion was to represent their work in Africa rather than her heritage. Both of them have been active on the continent and IIRC that is where he proposed.