Selenium is found in a few rare minerals such as crooksite and clausthalite. In years past it has been obtained from flue dusts remaining from processing copper sulfide ores, but the anode metal from electrolytic copper refineries now provide the source of most of the world's selenium. Selenium is recovered by roasting the muds with soda or sulfuric acid, or by smelting them with soda and niter.
Selenium exists in several allotropic forms.
Three are generally recognized, but as many as that have been claimed. Selenium can be prepared with either an amorphous or crystalline structure. The color of amorphous selenium is either red, in powder form, or black, in vitreous form. Naturally selenium contains six stable isotopes. Fifteen other isotopes have been characterized.