Final answer:
De Beers' previously near-total control over the diamond market has diminished due to increased competition, economic changes, and antitrust laws, yet it still holds a significant impact on the rough diamond market today.
Step-by-step explanation:
The dominance of the De Beers group in the global diamond market has seen significant shifts over the last century. Originally, De Beers controlled a virtual monopoly on the diamond trade. Their power began in the 19th century and was solidified by the end of the 1880s during the period known as 'the Scramble for Africa', where they, along with other European entities, took control of vast natural resources, including diamonds.
In more recent decades, however, this dominance has been challenged. New players and discoveries in other parts of the world, changes in global economic policies, antitrust laws, and a more fragmented distribution system have eroded their market share. Despite growing competition and a decline in their near-total control, De Beers remains a significant entity with considerable impact on the rough diamond market, maintaining extensive mining operations and a leading role in diamond distribution.