Small blanks are available for very reasonable prices. This glass is used primarily for large professional telescopes. ClearCeram-Z is a recently developed low expansion glass ceramic material from Ohara Corporation. This is a relatively recent innovation in mirror substrate materials mnufactred from sheets of fusion bonded and heat-treated plate glass and is manufactureed by ASM Products. The expansion coefficient slightly lower than pyrex, but much higher than the other glass ceramics.
Mirror blanks are inexpensive and readily available.
This category represents the cutting edge of technology. The materials are listed here for general interest only. All of these materials are either extremely expensive, difficult to work with or hazardous to work with. However, they do have uses as mirror materials in specialized applications may be available for use by amateurs in the future. The high coefficient of expansion makes aluminum unsuitable for amateur telescopes.