Silica occurs in living organisms. It is possible that silicon may have played an important, perhaps even necessary role, in the origin of life on the earth. The pattern of deposition of silica in plants is biologically specific and it is possible to identify plants by microscopic examination of silica particles. In some cases, silica appears to be a factor in the resistance that plants offer to diseases and insects.
Human tissues often contain from 6 to 90 mg of silica per 100 grams of dry tissues. Lung tissue may vary from 10 mg in infancy to as much as 2000 mg per 100 grams in old age. Miners, stonecutters, potters and others engaged in work where siliceous dust is breathed in large amounts often develop a serious lung disease called silicosis.