Modern quartz products can be many things to many people. The purest of quartz is silicon dioxide (SiO₂). Natural quartz has many impurities; some are good and some not so good for high technology use. For use in industry, science and communications, it is rarely pure SiO₂, but rather a mixture of SiO₂ with controlled trace impurities. Most of the trace impurities are introduced on purpose to give the quartz specific properties that it needs to do the task at hand.

The name used to describe the end-product may vary as much as the trace impurity amounts. Names like quartz, silica, fused silica, fused quartz, synthetic quartz, synthetic fused silica or synthetic fused quartz have been used in various publications, sometimes to describe the same product. Much of the terminology used in silicate glasses is inconsistent, and tends to be confusing when discussed on an introductory level. To help in this handbook, we will use the following definitions.

• Quartz is a natural grade of crystalline silicon dioxide (SiO₂). This is the most common phase of SiO₂. This is also referred to as “rock crystal.”
• Fused Quartz is a natural grade of amorphous SiO2. Typically produced from the melting (fusing) of crystalline quartz and refined such that an amorphous (glass) is formed.
• Silica is silicon dioxide (SiO₂).
• Fused Silica is silicon dioxide (SiO₂) in its amorphous (glassy) state.
• Synthetic Fused Silica is amorphous silicon dioxide that has been produced through chemical deposition rather than refinement of natural ore. This synthetic material is of much higher purity and quality as compared to fused quartz made from natural minerals.
• Doped (Synthetic) Fused Silica is amorphous silicon dioxide that has been produced through chemical deposition. It has been intentionally doped with trace elements such as germanium, fluorine, boron, phosphorous, titanium, etc. to adjust the optical properties of the glass.

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