Basalt

The young Russian geologist hammered off a piece of the stone and minutely examined it. "Basalt!" he cried rapturously, passing a fragment of the rock to his German colleague.

Basalt is a type of rock that forms when lava solidifies.

Black sand comes from eroded volcanic material such as lava, basalt rocks, and other dark-colored rocks and minerals, and is typically found on beaches near volcanic activity.

He calls it clear evidence that water carried the salts up from basalt rock below ground and left them behind on the surface when it evaporated.

A connection between microbial activity and the appearance of microtubules in basalt glass has been proposed in the past, but direct evidence for biological processes being in their formation is still lacking and non-biological origins are possible.