silica vs. silicon

the dioxide form of silicon, SiO 2 , occurring especially as quartz sand, flint, and agate: used usually in the form of its prepared white powder chiefly in the manufacture of glass, water glass, ceramics, and abrasives.

a nonmetallic element, having amorphous and crystalline forms, occurring in a combined state in minerals and rocks and constituting more than one fourth of the earth's crust: used in steelmaking, alloys, etc. : Si; : 28.086; : 14; : 2.4 at 20°C.