The most stable variety, crystalline hexagonal selenium, is a metallic grey colour. Crystalline monoclinic selenium is deep red. Source Most of the world. These muds are either roasted with soda or sulfuric acid, or smelted with soda to release the selenium. Selenium is found in a few rare minerals.
Selenium is therefore useful in photocells and solar cells.
It can also convert a.c. electricity to d.c. electricity, so is extensively used in rectifiers. It is used by the glass industry, and to make stainless steel. It is also used in photocopiers. Biological Role Selenium is an essential trace element but is toxic in excess. It is carcinogenic and teratogenic.