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Silicon Definition & Meaning - Merriam-Webster

Silica is a chemical compound made of silicon and oxygen with the formula SiO2. It can be found in many forms, including crystalline silica and amorphous silica, and is present in the environment in sand, soil, rocks, water, and air. 

Silica can also be found in concrete and bricks, and in the form of silicone polymers it can be found in sealants, lubricants, and even medical applications like surgical implants. 

Why aren't people or other living things silicon based?  It's due to the fact that when silicon reacts with oxygen, it forms a stable lattice which is then not possible to easily eliminate from the "organism" (if the organism did indeed exist), and so a silicon-based life form would not be able to get rid of the waste products arising from its own metabolism.