Silicon, silica, silicates and silicone have similar names but are different things. Keeping them straight avoids misunderstandings when reading material data.

Silicon

By crustal abundance silicon ranks second among the elements, at roughly 27% of the crust's mass; only oxygen is more plentiful. In nature it is almost always combined with oxygen, as silica or as silicates.

Silica and silicates

Compounds of silicon and oxygen are broadly called silicon-oxygen compounds. Quartz is a mineral made of pure silicon dioxide, and tridymite, coesite and cristobalite are other mineral forms of silicon dioxide. Feldspar is different: besides silicon and oxygen it contains sodium, aluminum, potassium and calcium, so feldspar is a silicate but not pure silica.

Geology adds one more source of confusion. When a rock is analyzed, its silicon is reported as the equivalent amount of silicon dioxide, so you may hear that a rock is 48% silica even though no quartz can be seen in it. That is like baking a cake: the eggs are in the recipe but you cannot find one in the finished cake. The silicon and oxygen are present, but they have combined with other elements into minerals such as feldspar.

Silicone

Silicone is a synthetic polymer with a backbone of alternating silicon and oxygen atoms and carbon-containing organic side groups. It can be a solid, a liquid or a gel, and it has many medical uses, but it is not something found in rock.

Native silicon

Elemental silicon does occur in nature, but very rarely. Russian geologists once collected volcanic gas hotter than 900°C using quartz tubes, and the minerals that deposited in the tubes included metallic silicon embedded in rock, in crystals up to about 0.3 mm across, as a silicon-aluminum alloy. In everyday life, what you meet is silica and silicates, not elemental silicon, and certainly not silicone.