Vug

A vug, vugh, or vugg (/ˈvʌɡ/)[1] is a small- to medium-sized[quantify] cavity inside rock.

Most commonly, cracks and fissures opened by tectonic activity (folding and faulting) are partially filled by quartz, calcite, and other secondary minerals.

Open spaces within breccias formed by an ancient collapse are another important source of vugs.

[citation needed] Vugs may also form when mineral crystals or fossils inside a rock matrix are later removed through erosion or dissolution processes, leaving behind irregular voids.

[2] The word vug was introduced to the English language by Cornish miners, from the days when Cornwall was a major supplier of tin.