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A vug, vugh, or vugg (/vʌɡ/)^[1] is a small- to medium-sized^[quantify] cavity inside rock. It may be formed through a variety of processes. 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. The inner surfaces of such vugs are often coated with a crystal druse. Fine crystals are often found in vugs where the open space allows the free development of external crystal form.

The term vug is not applied to veins and fissures that have become completely filled, but may be applied to any small cavities within such veins. Geodes are a vug-formed rock, although that term is usually reserved for more rounded crystal-lined cavities in sedimentary rocks and ancient lavas.^[2]

The word vug was introduced to the English language by Cornish miners, from the days when Cornwall was a major supplier of tin.^[3] The Cornish word was vooga, which meant "cave".^[3]

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  A vug in a limonite matrix is the host for a layer of green botryoids of conichalcite and colorless, translucent rhombs of calcite
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  Vug with rosettes of deep blue azurite on a field of malachite
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  Goosecreekite with heulandite-Ca on quartz
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  Intergrown botryoids of blue-green rosasite inside the curve of a limonite vug
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  Example of open space in a vug allowing the formation of crystallized minerals
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  Apple green pyromorphite cluster on limonite matrix
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  Vug in a limonitic matrix is the host for bi-pyramidal, brownish-red crystals of wulfenite
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  Crystallized groutite scattered about a hollow vug
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  In a vug of limonite are intergrown clusters of rhombohedral calcite with inclusions of fibrous aurichalcite
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  Stellerite, epistilbite, and eluorapophyllite in a basaltic vug
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  Nestled in its matrix vug are five golden-yellow crystals of sulfur
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  In an open pocket in a matrix of limonite there is a cluster of deep robin's-egg blue chrysocolla with a covering of drusy quartz

• Amygdule
• Rock microstructure
• Vesicular texture

• The dictionary definition of vug at Wiktionary