Other forms: vugs
A vug is a small cavity, or empty space, inside a rock. A vug may or may not be lined with crystals.
The word vug comes from the Cornish word for "cave," vooga. (Cornish is a Celtic language that used to be spoken in Cornwall, a region in southwestern England, but it died out around 1800.) Unlike a geode, a vug does not necessarily have a spherical shape, nor is it always lined with crystals. But vugs can have beautiful crystal formations, such as quartz or calcite. Natural history or geological museums often display vugs.