Final answer:
The first diamond found in South Africa, a 21.25 carat yellow stone, is called the Eureka Diamond. Its discovery marked the start of diamond mining in the region and led to significant economic and political changes.
Step-by-step explanation:
The 21.25 carat yellow diamond, which was the first diamond found in South Africa, is known as the Eureka Diamond. Its discovery near the Orange River in 1867 marked the beginning of diamond mining in the region, eventually leading to the establishment of the DeBeers company. This development not only had significant economic implications but also directly influenced geopolitical dynamics, such as Britain's involvement in the area and the subsequent Boer Wars for control over South Africa's precious mineral resources.