Natural sand from Sperrgebiet, Namibia (the name is from the German kolonial period). This area is known for its diamonds, and there is still active mining.
Real diamonds occur in their natural octahedronm or cubic crystal shape. But these can be eroded to form othe more rounded shapes. Keep an eye on grains that look different.
Betst method for finding diamonds is to spread a little material on e white surface, and to examine the sand under a microscope or loupe. Best to have tweezers and a bottle ready in case you find something. It is also very interesting to studie the other minerals in this sand.