Vug

In the lower left, a spherule, or sphere-shaped grain, can be seen penetrating the interior of a small cavity called a vug.

Vugs are small cavities inside rock that are formed when crystals form inside a rock matrix and are later removed through erosive processes, leaving behind voids. A common cause of vugs is minerals precipitating from solution in water, and then later being dissolved again by less saturated water. The inner surfaces of vugs are often coated with some of the mineral matter that formed them.

Vug is also refers to a Titanian individual in Philip K. Dick's science fiction novel The Game-Players of Titan, ISBN 0679740651.