K9 glass
K9 glass, sometimes referred to as K9 crystal, is a variety of optical borosilicate crown glass. The letter "K" is a reference to the German word for "crown" and the number 9 refers to the lead oxide content of the glass. K9 glass has high optical clarity, and is used in many contexts requiring this characteristic. It has been regarded as a difficult-to-cut material because of its superior mechanical properties. It has an elastic modulus per pascal of 8x1010, a Poisson's ratio of 0.21, and a density of 2510 kilograms per cubic meter. Its refractive index is 1.517 at 590 nm wavelength Its clarity, combined with low cost have made it desirable for chandeliers, lasers, telescopes, etc.
K9 is produced in large quantities by China, which sells it at a price far below higher-quality well-known glass manufacturers such as Swarovski. A similar product to K9 is BK7.