Substance derived from silica (sand is a silica). The unique fluid properties of silicone give it a great deal of slip, and in its various forms it can feel like silk on the skin, impart emolliency, and be a water-binding agent that holds up well, even when skin becomes wet. In other forms, it is also used extensively for wound healing and for improving the appearance of scars (Source: Journal of Wound Care, July 2000, pages 319–324).
There are numerous forms of silicones used in cosmetic products, particularly leave-on skin-care products and all manner of hair-care products. Perhaps the most common forms of silicone are cyclopentasiloxane and cyclohexasiloxane. Other forms include various types of dimethicone and phenyl trimethicone.
Claims that silicones in any form cause or worsen acne have not been substantiated in published research, nor have reports that silicones are irritating or "suffocate" skin. Almost all of these claims are either myths or based on anecdotal evidence, which isn't the best way to determine the safety or efficacy of any cosmetic ingredient.
How do we know that silicones don’t suffocate skin? Because of their molecular properties they are at the same time porous and resistant to air. Think of silicones in a skin-care formula like the covering of a tea bag. When you steep the tea bag in water the tea and all of its antioxidant properties are released. Silicones remain on the surface of your skin and the other ingredients it is mixed with "steep" through. All ingredients have to be suspended in some base formula. Some of those ingredients remain on the surface some absorb. Either way the "actives" get through. Think of how many topical medications are suspended in petrolatum or mineral oil and those active ingredients absolutely get through and petrolatum is far more effective at preventing moisture loss than silicones are. Silicones have been used in burn units for years because of their unique healing, protecting, and breathable properties.
Moreover, the molecular structure of commonly used silicones makes it impossible for them to suffocate skin. The unique molecular structure of silicones (larger molecules with wider spaces between each molecule) allow them to form a breathable barrier and also explains why silicones rarely feel heavy or occlusive, although they offer protection against moisture loss (Source: The Chemistry and Manufacture of Cosmetics, Volume 3, Book 2, Allured Publishing Corporation, 2002, pages 833-839).
What about silicones clogging pores? Interestingly, silicone has been shown to be helpful for offsetting dryness and flaking from common anti-acne active ingredients such as benzoyl peroxide and topical antibiotics (Source: Cutis, October 2008, pages 281-284). Also, silicone fillers are sometimes used for improving the appearance of acne scars. That certainly wouldn't be the case if silicone was a pore-clogging ingredient (Source: Dermatology Research and Practice, October 2010, Epublication).
Perhaps the most telling reason why silicones do not clog pores and cause acne (or blackheads) is because, from a chemistry standpoint, most silicones are volatile substances. That means their initially viscous (thick) texture evaporates quickly and does not penetrate the pore lining where acne is formed. Instead, they help ensure an even application of other ingredients and leave behind a silky, almost imperceptible feel that noticeably enhances skin's texture and appearance. You can think of this as a breathable barrier that protects skin while barely being felt.