In nature, selenium is found in the Earth's crust, 0.09 ppm, usually in sulfide ores of heaby metals but also in minor amounts in minerals such as pyrite, clausthalite, naumannite, tiemannite and selenosulfur . The isotopes found in nature are 74Se (0.87 %, stable), 76Se (9.02 %, stable), 77Se (7.58%, stable), 78Se (23.52 %, stable), 80Se (49.82 %, stable) and 82Se (9.19 %, stable) .
In pure solid form, selenium exist exist as:
Vitreous amorphous form 
Red monoclinic α-form .
Red monoclinic β-form .
Grey/metallic hexagonal crystal .