Tridymite and cristobalite phases of silica. Ordinary kiln wash made from a mixture of kaolin and quartz can produce significant amounts of very fine grained cristobalite. This poses a greater health risk than ordinary quartz.
In ceramics, cristobalite is a form (polymorph) of silica. During firing quartz particles in porcelain can convert to cristobalite. This has implications on the thermal expansion of the fired matrix.