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 to medium-sized cavities inside rock that may be formed through a variety of processes. In Geology, rock is a naturally occurring aggregate of Minerals and/or Mineraloids The Earth's outer solid layer the ‘ Lithosphere Most commonly cracks and fissures opened by tectonic activity (folding and faulting) are partially filled by quartz, calcite, and other secondary minerals. See also Folding The term fold is used in Geology when one or a stack of originally flat and planar surfaces such as Sedimentary In Geology a fault, or fault line, is a planar rock fracture which shows evidence of relative movement Quartz (from German) is the most abundant Mineral in the Earth 's Continental crust (although Feldspar is more common in Calcite is a carbonate mineral and the most stable polymorph of Calcium carbonate ( Ca[[carbon C]] O 3 A mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition a highly ordered atomic structure and specific Open spaces within ancient collapse breccias are another important source of vugs. Breccia (ˈbrɛtʃiə ˈbrɛʃiə breach is a rock composed of angular fragments of several Minerals or rocks in a matrix, that is a cementing material Vugs may also result when mineral crystals or fossils inside a rock matrix are later removed through erosion or dissolution processes, leaving behind irregular voids. In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating The inner surfaces of such vugs are often coated with some of the mineral matter that formed them. 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 common vug formed rock, although that term is usually reserved for more rounded crystal-lined cavities in sedimentary rocks and ancient lavas. Geodes ( Greek geoides, "earthlike" are geological rock formations which occur in sedimentary and certain Volcanic Sedimentary rock is one of the three main rock types (the others being igneous and Metamorphic rock) Lava is molten rock expelled by a Volcano during an eruption When first expelled from a volcanic vent it is a Liquid at Temperatures

The word vug was introduced to the English language by Cornish miners, from the days when Cornwall was a major supplier of tin. Cornwall ( Kernow ˈkɛɹnɔʊ is the most southwesterly county of England, on the Peninsula that lies to the west of the River Tamar Mining is the extraction of valuable Minerals or other geological materials from the earth usually (but not always from an Ore body Cornwall ( Kernow ˈkɛɹnɔʊ is the most southwesterly county of England, on the Peninsula that lies to the west of the River Tamar Tin is a Chemical element with the symbol Sn (stannum and Atomic number 50 The Cornish word was vooga which meant "cave. For the Cornish-English dialect see West Country dialects and List of Cornish dialect words. " The word can also be spelled as vugg and/or vugh.