In the 1970s , two minor Cullinan diamonds owned by Louis Botha 's heirs were analysed by De Beers at their laboratory in Johannesburg .
Heirs were found to be completely free of nitrogen or any other impurities .
In the 1970s , two minor Cullinan diamonds owned by Louis Botha 's heirs were analysed by De Beers at their laboratory in Johannesburg , and they were found to be completely free of nitrogen or any other impurities .