Sapphire Windows withstand high pressures.
Press Release Summary:

Original Press Release:
Sapphire Windows Withstand Heat And High Pressure
Custom engineered sapphire windows that are designed to be used in a wide range of vacuum furnace, chemical processing, and industrial applications are available from Meller Optics, Inc. of Providence, Rhode Island.
Meller High-Strength Sapphire Windows withstand high pressures and are insoluble in water and common acids and alkalis to approximately 1,000C. Suitable for a wide range of applications in hostile environments, they can be custom fabricated in sizes from '4' to 6" dia. and up to 1.5" thick.
Featuring Moh 9 hardness, Meller High-Strength Sapphire Windows measuring 2.7" dia. x 1.5" thick are capable of withstanding up to 18,000 psi. Coated windows are ideal for providing optical access into a furnace, chemical processing, and general vacuum and high pressure equipment.
Meller High-Strength Sapphire Windows are priced according to size and quantity. Literature and price quotations are available upon request.