English: Boron is not a very frequent element and is found in nature only in compounds together with oxygen. Crystalline boron, which is shown here, is nearly as hard as diamond (9.5 on Mohs scale, diamond has 10). Boron compounds have many, often very special applications, a common one is B₂O₃ for borosilicate glass. Most famous probably are the perborates as bleach and as washing agent. Because of their bad biodegradability, the use of perborates has gone back.