Workers are demanding a 15 percent increase, against an offer from De Beers for 7.5 percent with a once-off bonus of 2,500 rand ($362, 255 euros). Inflation in South Africa was at 5.0 percent in June.

Unions complain that basic expenses like food and electricity have been rising far faster than general inflation, with the national power utility imposing a series of annual rate hikes of about 25 percent.

“Food and fuel inflation have skyrocketed and the bulk of the workforce walk to work on empty stomachs. The (union) appeals to the company to reconsider its position and avoid the crippling mass action,” the union said in a statement Wednesday.

The mid-year winter months are known as “strike season” in South Africa, where contract negotiations around the end of the fiscal year on June 30 are routinely marked by strike calls from the politically powerful unions.

De Beers is 45 percent owned by global mining giant Anglo American, 40 percent by the Oppenheimer family and 15 percent by the government of Botswana.