Zerodur® is a carefully chosen combination of glass with a positive expansion coefficient and a form of quartz with a negative expansion coefficient.
The mixture has a near zero coefficient of thermal expansion. Due to very little transmission near the blue end of the visible spectrum Zerodur® is rarely used for lenses or prisms but it is an ideal backing for a large parabolic mirror.
Note: the more common zero expansion material in domestic use is pyrex. Pyrex is a mixture of two glasses - one of which contains boron and has a negative coefficient of linear expansion. If the mixture is in the right proportions the final material has a near zero coefficient of expansion. Pyrex is used for 'oven proof' dishes that would crack if made from ordinary glass.
Fused quartz has a low coefficient of linear expansion. Quartz fiber can be melted into tiny balls with a gas flame. The red hot balls sizzle when dropped into cold water, but they survive intact!