Vitrified sand
Vitrified sand is sand that has been heated to a high enough temperature as to partly or fully melt the silicon dioxide or quartz that is the main ingredient of common sand. When sand is used to make glass, soda ash or potash are added to lower the melting point. Pure quartz melts at. 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.
