Firestone Diamonds (LON:FDI) said Angus Ogilvie has resigned from the board as finance director to pursue other interests.
The company expects to announce the appointment of a new chief financial officer in due course.
Non-executive director Braam Jonker will oversee the finance function of the company until then.
Chief executive Tim Wilkes said: "On behalf of the board, I would like to thank Angus for his contributions to Firestone and wish him the best with his future endeavours."
Firestone is a miner and explorer with operations focused on Lesotho and Botswana.
Firestone currently operates the Liqhobong mine in Lesotho and is also the largest holder of mineral rights in Botswana's diamondiferous kimberlite fields, controlling approximately 10,000 square kilometres around the major Orapa and Jwaneng mines and the entire Tsabong kimberlite field.
In addition to Liqhobong and the BK11 mine in Botswana, Firestone has 108 kimberlites in its portfolio, of which 30 have been proven to be diamondiferous.