Vitrified sand is sand that has been heated to a high enough temperature as to partly melt the silicon dioxide

Silicon dioxide

The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...

 or quartz

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

 that is the main ingredient of common sand. When sand is used to make glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

, soda ash or potash

Potash

Potash is the common name for various mined and manufactured salts that contain potassium in water-soluble form. In some rare cases, potash can be formed with traces of organic materials such as plant remains, and this was the major historical source for it before the industrial era...

 are added to lower the melting point. Pure quartz melts at 1650°C. There are several natural processes that produce more or less melted sand and one man-made form:

• fulgurite
  Fulgurite
  Fulgurites are natural hollow glass tubes formed in quartzose sand, or silica, or soil by lightning strikes. They are formed when lightning with a temperature of at least instantaneously melts silica on a conductive surface and fuses grains together; the fulgurite tube is the cooled product...
  
   is sand fused by a lightning bolt hitting sand.
• tektite
  Tektite
  Tektites are natural glass rocks up to a few centimeters in size, which most scientists argue were formed by the impact of large meteorites on Earth's surface. Tektites are typically black or olive-green, and their shape varies from rounded to irregular.Tektites are among the "driest" rocks, with...
  
   is sand fused by the compressive heat of a meteor strike including moldavite
  Moldavite
  Moldavite is an olive-green or dull greenish vitreous substance possibly formed by a meteorite impact. It is one kind of tektite. It was named by A. Dufrnoy for the town of Moldauthein in Bohemia , where it occurs...
  
   a gem quality stone mined in some locations.
• frit
  Frit
  Frit is a ceramic composition that has been fused in a special fusing oven, quenched to form a glass, and granulated. Frits form an important part of the batches used in compounding enamels and ceramic glazes; the purpose of this pre-fusion is to render any soluble and/or toxic components insoluble...
  
   is partially fused sand plus other chemicals in the process of glaze and glass making.
• trinitite
  Trinitite
  Trinitite, also known as Atomsite or Alamogordo Glass, is the name given to the glassy residue left on the desert floor after the plutonium-based Trinity nuclear bomb test on July 16, 1945, near Alamogordo, New Mexico...
  
   vitrified sand forms below nuclear blasts close to a sand surface. Examples include, Trinity (nuclear test) and Emu Field
  Emu Field
  Emu Field is located in the desert of South Australia, at . Variously known as Emu Field, Emu Junction or Emu, it was the site of the Operation Totem pair of nuclear tests conducted by the British government in October 1953.The site was surveyed by Len Beadell in 1952...
  
  .

Volcanic glass

Volcanic glass

Volcanic glass is the amorphous product of rapidly cooling magma. Like all types of glass, it is a state of matter intermediate between the close-packed, highly ordered array of a crystal and the highly disordered array of gas...

in many forms is not fused sand because it is formed from molten material.