Selenium is a red or gray powder, a massive quasi metal with a gray metallic luster. Of the six known solid allotropes, three crystals（ α Single Italic β Monoclinic, and gray triangular crystals) are the most important. The gray hexagonal crystal system is the most stable among the crystals, with a density of 4.81 g/cm3. It also exists in three amorphous solid forms: red and black amorphous glass like selenium. The former is brittle with a density of 4.26g/cm3; The latter has a density of 4.28 g/cm3, and the other is colloidal selenium.