Fused quartz and fused silica are types of glass containing primarily silica in amorphous (non-crystalline) form. They are manufactured using several different processes. Note that glasses formed by the traditional 'melt-quench' methods (heating the material to melting temperatures, then rapidly cooling to the solid glass phase), are often referred to as 'vitreous', as in 'vitreous silica'. The term 'vitreous' is synonymous with 'glass', when used in the melt-quench context. Fused quartz is manufactured by melting naturally occurring quartz crystals of high purity at approximately 2000°C, using either an electrically heated furnace (electrically fused) or a gas/oxygen-fuelled furnace (flame fused). Fused quartz is normally transparent. Fused quartz can also form naturally. The naturally occurring form is usually referred to as Metaquartzite and is formed under metamorphic conditions. An increase in heat causes the crystals within the quartz to become fused together. Fused silica is produced using high purity silica sand as the feedstock, and is normally melted using an electric furnace, resulting in a material that is translucent or opaque.

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