As the first link states:

  Errol said his children grew up watching him sell emeralds all over the world, after he had them cut in Johannesburg. However, he always stuck to the rules of the trade: contact the potential buyer, meet in a neutral public place ("for obvious reasons"), and be subtle about exchanging money for gems.

Errol Musk had multiple lines of income, including NSRs from three emerald mines that made a return.