Vitrified sand is sand that has been heated to a high enough temperature to undergo vitrification, which is the melting of the silicon dioxide or quartz that compose common sand.
Vitrified sand is a type of natural glass, contrasted with manufactured glass in which soda ash or potash are added to lower the melting point.
Pure quartz melts at 1,650 °C (3,002 °F).
There are several natural processes that produce more or less melted sand and one man-made form: Volcanic glass in many forms is not fused sand because it is formed from molten material.